The report, which covers the month of April, shows that the overall leader in total minutes, Facebook, grew an astounding 699 percent-- 13.9 billion minutes this April, compared to 1.7 billion minutes in April last year.

'Remember Friendster?'

The number-two site, MySpace, saw a 31 percent drop, while Twitter has rocketed up to sixth place with 3,712 percent growth.

Jon Gibs, online media vice president at Nielsen online, said the report indicated "regardless of how fast a site is growing or how big it is, it can quickly fall out of favor" with users.

"Remember Friendster?" he asked, mentioning a social-networking site that isn't in this year's top 10. "Remember when MySpace was an unbeatable force?"

He added that Facebook and Twitter shouldn't take their positions for granted, since consumers have clearly shown that "they are willing to pick up their networks and move them to another platform, seemingly at a moment's notice."

The report also showed that while Facebook remains in the top position for unique visitors and total minutes for the fourth month straight, MySpace is the leader in online video. In terms of number of streams and total minutes watching video, MySpace's visitors spent 384 million minutes watching more than 120 million streams, for nearly 39 minutes per visitor.

Because of this disparity in the kinds of leads different sites have, Gibs said it might be better not to ask who is winning, but "what audiences are they drawing, and how are they building for the future" to maintain the loyalty of fickle users.

'Auras' for Sites

Brad Shimmin, an analyst with Current Analysis, said the trends in site popularity are creating what he described as "auras" for specific sites -- their brand characteristics. "MySpace," he said, "is more about something I did and saw, while Facebook is 'Here's who I know.'"

The average age of a site's users is also changing, he noted. For instance, Shimmin said, "we always thought LinkedIn would be the one for adults," but Facebook has begun to skew older as more adults put up pages.

The reasons for the ebbs and flows in a site's popularity can be hard to pinpoint, but Shimmin suggested that MySpace has diminished in part because of its "clutter problem," where looking at someone's profile can become annoying because "too much is happening."

Shimmin predicted that the "federating of personal data " through networks of sites that use such protocols as Facebook Connect or OpenSocial could "begin to mitigate the 'jumping ship' behavior of some users." But, he added, one thing that could become more pronounced are cultural differences between countries, such as the British being more obsessed with texting than Americans, or Asia's highly popular Hi5 site being almost unknown in the U.S.