A website makes you a deal: Pay $25, and they'll give you $50 worth of food at your favorite local restaurant. Sign me up, right? But wait, there's a catch: You need to convince as many of your friends as possible (and your friends' friends) to agree to this deal, because unless 100 people are interested, it's a no-go. Oh, and you have 24 hours to make this happen. \n\nThat's what people are calling "social buying"\u2014an idea that was originally born during the dotcom boom but failed to gain traction. Now, with the help of social networks like Facebook and Twitter to help spread the word\u2014and thanks to the dismal economy\u2014social buying sites are gaining steam. \n\n[Learn how you can keep your social reputation intact online: "Managing Your Reputation Online: 5 Essential Tools."] \n\nMany sites are geared toward a major city and offer area residents steep discounts on everything from food to events to spa packages and more. Many set a time limit on the offer and require a certain number of people to join the deal\u2014a number that's agreed upon by the social buying site and the business offering the service. Once enough people sign up, the deal is valid, and everyone receives a coupon. \n\nWant in? Here's a list of 10 social buying sites. Some, such as Groupon, are well-established and deliver discounts to several cities nationwide. Other sites have just launched and feature a limited number of cities, so be sure to check back if your location isn't yet available. \n\n1. BuyWithMe\n\n\n\nBuyWithMe deals are often 50 percent off or more. Depending on the city, each deal is usually offered for a week. Boston deals feature something new several times a week while San Diego usually selects one per week. BuyWithMe also just secured $5.5 million in funding. \nAvailable cities: Boston, San Diego, Washington D.C.\u2014coming soon to more cities. \nPast deals: $50 for $100 of spa services; $10 for three hours of snow tubing; $39 for a dozen premium roses ($85 value).\n\n2. eWinWin \n\n\n\neWinWin differs from other social buying sites because it does not require a certain number of people to "tip" a deal; rather, a discount is offered from the start, and as more buyers join, the discount increases. You can expect savings of up to 70 percent.\nAvailable cities: Tampa, Fla. There are three "buying groups" that offer deals around specific areas. Right now, they're centered around the Florida State University campus, University of South Florida campus and University of Florida campus.\nPast deals: $5 for two 22oz. smoothies; $11 for an oil change ($25 value); 50% off sushi and sake.\n\n\n\n3. Groop Swoop \n\n\n\nGroop Swoop was launched in October and focuses on discounts in the San Francisco and East Bay areas. New deals pop up every day. Like many other sites, Groop Swoop requires a minimum number of participants to make a deal valid. \nAvailable cities: San Francisco, East Bay area.\nPast deals: $10 for $20 worth of Northern Indian cuisine; save 66% on two weeks of exercise boot camp training; $40 for a one-hour facial (save 56%).\n\n\n4. Groupon \n\n\n\nGroupon, arguably the leader in social buying, boasts branches representing 49 cities (although a bit more than half are currently active). Groupon was launched in November 2008 and raised $30 million in financing in December 2009. Groupon features a new deal every day; generally, the offer expires at midnight. Also, if you refer someone to the site and he or she subscribes within 72 hours, you'll receive a $10 credit when the person makes the initial Groupon purchase.\nAvailable cities: Albuquerque, Atlanta, Austin, Baltimore, Boston, charlotte, Chicago, Cincinnati, Cleveland, Columbus, Dallas, Denver, Detroit, Fort Worth, Fresno, Houston, Indianapolis, Jacksonville, Kansas City, Las Vegas, London, Los Angeles, Louisville, Memphis, Miami, Milwaukee, Minneapolis\/St. Paul, Nashville, New York, Oklahoma City, Omaha, Philadelphia, Phoenix, Pittsburgh, Portland, Raleigh\/Durham, Sacramento, San Antonio, San Diego, San Francisco, San Jose, Seattle, Sioux Falls, St. Louis, Tampa, Toronto, Vancouver, Virginia Beach, Washington D.C.\nPast deals: $45 truffle workshop ($88 value); $45 restorative massage ($100 value); $60 for a helicopter ride ($120 value).\n\n5. LivingSocial \n\n\n\nLivingSocial is the company behind the Facebook app "Visual Bookshelf" and its self-titled social networking site. Last year, LivingSocial jumped into social buying, and now dishes up discounts of more than 50 percent for local restaurants, spas, hotels, sporting events and more. \nAvailable cities: Atlanta, Austin, Boston, New York, San Francisco, Seattle, Washington D.C.\nPast deals: 50% off sushi-making classes; 42% off Boston Celtics tickets; 55% off luxury bus tickets between New York and Washington D.C.\n\n\n\n6. Play @ Savvy Circle \n\n\n\nPlay launched in 2007 as a site to help people save money while shopping, then delved into social buying in Hawaii, and is looking to branch out to other states. Play also has a special feature called "prePLAY" in which you gather a group of people to persuade a merchant to give you a deal, though there is no guarantee that a deal will be agreed upon.\nAvailable cities: Honolulu; more locations to come.\nPast deals: $25 for $50 worth of food at a diner; $29 for a $55 ticket to Stomp; $35 for two 30-minute Shiatsu sessions ($70 value).\n\n7. Pricebunch \n\n\n\nPricebunch says it features a deal every day that becomes valid once a minimum number of buyers sign on. However, it appears that some deals are featured for longer than one day. Like the Groupon referral incentive, Pricebunch offers a $10 credit to your account when someone you refer to the site completes his or her first purchase. \nAvailable cities: Boston, Chicago, Los Angeles, San Francisco, Washington D.C. Right now, it appears that the Chicago and Los Angeles sites are the only ones with current deals.\nPast deals: $20 for $40 worth of cake and pastries; $14 for a whale watch.\n\n\n8. Qponus \n\n\n\nQponus was started in Boston last year, and now offers users a deal everyday. Qponus, too, requires a minimum number of buyers for a deal to be valid. The duration of each deal varies.\nAvailable cities: Boston, Philadelphia. \nPast deals: 54% off personal training and evaluation; save 50% on a UV-free airbrush spray tan; 40% off cross-country ski lessons.\n\n\n\n9. SocialBuy \n\n\n\nSocialBuy launched in beta just last week, and is offering deals in Los Angeles and San Francisco. Its daily deals feature products from such brands as PinkBerry, Sprinkles Cupcakes and other local retailers.\nAvailable cities: Los Angeles and San Francisco. SocialBuy has plans to branch out to San Diego, San Jose, Las Vegas, Phoenix, Seattle and Portland.\nPast deals: $25 for $50 worth of flower arrangements; $39 for a one-hour gondola cruise ($80 value); $25 for $50 worth of services at Lash Lab.\n\n\n10. TownHog \n\n\n\nTownHog appears to be in its infancy as it boasts several cities, however seems as if San Francisco is the only city with live deals. (The site does say that it's in the process of adding additional cities, but it depends on interest in TownHog. The more people who sign up for it in a given city, the higher the likelihood the site will go live there.) Like the other sites, TownHog features discounts on fitness classes, spa services and more, and requires a minimum number of buyers.\nAvailable cities: San Francisco, although more are promised.\nPast deals: $45 for $100 worth of flower arrangements; half-off a spa pedicure; $30 for one month of unlimited yoga classes ($88 value); $20 for $40 worth of gourmet cream puffs.\n\n\nStaff Writer Kristin Burnham covers consumer Web and social technologies for CIO.com. She writes frequently on Twitter, Facebook, LinkedIn and Google. You can follow her on Twitter: @kmburnham.