CIO — 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.

That's what people are calling "social buying"—an 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—and thanks to the dismal economy—social buying sites are gaining steam.

[Learn how you can keep your social reputation intact online: "Managing Your Reputation Online: 5 Essential Tools."]

Many 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—a 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.

Want 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.

1. BuyWithMe

BuyWithMe 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.
Available cities: Boston, San Diego, Washington D.C.—coming soon to more cities.
Past deals: $50 for $100 of spa services; $10 for three hours of snow tubing; $39 for a dozen premium roses ($85 value).

2. eWinWin

eWinWin 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.
Available 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.
Past deals: $5 for two 22oz. smoothies; $11 for an oil change ($25 value); 50% off sushi and sake.

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