Howell says he wonders what size network LinkedIn can handle. And, according to LinkedIn's Duncan, the service is optimized for networks with hundreds of people. The LinkedIn philosophy, he adds, emphasizes the quality of the people inside a member's network rather than the quantity. "We believe that 99 percent of LinkedIn users are going to be people who are looking to optimize the quality of their network," he says. "That said, there are always going to be people on LinkedIn who have 20,000 or 30,000 connections. While we recognize that these people are our most active users and they're paying for features, we have to balance them with the other 99 percent of users."

Duncan notes that there's more to the challenge of supporting millions of members with networks of hundreds and thousands of connections than just storage. Mapping connections and degrees of separation between members is computationally intensive, too, he says.

The problems Howell has experienced, however, aren't isolated to users with thousands of connections. Kerry Duggan, a campaign manager with the League of Conservation Voters in Washington, D.C., has 226 contacts in her network, and says that for a month she couldn't import e-mail addresses from her Gmail account to LinkedIn. She says LinkedIn's import feature is really helpful "when it works," but when it didn't, it wasn't too much of a pain. "I recommended LinkedIn to my brother, a wealth manager, and he's having problems importing from Hotmail, which is unfortunate because in both of our fields, networking is key."

"We don't feel that there's a huge number of problems with our existing functionality," says Duncan. "When we look through our reports to see the number of people accepting and sending invitations, we get the sense that the vast majority of users are having a good experience. That said, we're always going to continue to try to improve the experience for users."