"The one thing that is clear about social networking is that regardless of how fast a site is growing or how big it is, it can quickly fall out of favor with consumers," said Jon Gibs, vice president of media and agency insights at Nielsen Online. "Remember when MySpace was an unbeatable force?" added Gibs. "Neither Facebook nor Twitter are immune ... Consumers have shown that they are willing to pick up their networks and move them to another platform, seemingly at a moment's notice."

Facebook's rise to the top of the social networking food chain has been dramatic. In the past, total minutes spent on the site increased nearly 700% -- growing from 1.7 billion minutes in April 2008 to 13.9 billion in April 2009, according to Nielsen.

During that same time period, however, a possible successor to the throne has emerged, according to Gibs: "Twitter has come on the scene in an explosive way, perhaps changing the outlook for the entire space."

So far, Facebook is also failing to corner the critical arena of Web video. Indeed, while April was the fourth month in a row that it held the top spot in both unique visitors and total minutes, MySpace has been winning the Web video game.

With 120.8 million video streams, Nielsen finds that MySpace was the top social networking destination when ranked by streams and total minutes spent viewing video.

MySpace visitors spent 384 million minutes viewing video on the site, with an average of 38.8 minutes per viewer.

By comparison, Facebook visitors spent only 113.5 million minutes viewing video in April, with an average of 11.2 minutes per video viewer, according to Nielsen.

Overall, total minutes spent on social networking sites by U.S. consumers increased 83% year-over-year.

"Maybe the better question to ask is who goes to each site reach -- not who is 'winning,'" noted Gibs. "What audiences are they drawing and how are they building for the future to maintain the loyalty of their visitors -- who, to this point, have shown little long-term loyalty to any specific platform."