When Facebook redesigned its homepage in early March in a wholehearted embrace of the real-time activity stream as its primary user interface, everybody complained. “Why on earth does the world need 2 Twitters?,” asked one of my friends on Facebook. Twitter-envy aside, some early data suggests that embracing the stream was the right decision after all.

Since the redesign went into effect, Facebook’s growth has accelerated. After flat 0.3 growth in February, Facebook added nearly 4 million unique U.S. visitors in March (up 6.6 percent over February), and another 5 million in April (up 10.3 percent over March) to end at 67.5 million domestic uniques, according to comScore. That puts it within kissing distance of MySpace’s 71 million unique U.S. visitors in April, by the way (see chart below), and keeps a healthy 50-million visitor gap with Twitter, which added 8 million U.S. visitors in April alone.

Facebook is not alone when it comes to wading in the stream. AOL has found religion as well and is injecting “lifestreams” into everything from Bebo to Socialthing to AIM.

On the other end of the spectrum, is tiny FriendFeed, which is one big activity stream. Despite leading on the innovation front, FriendFeed has been struggling when it comes to gaining users. But a redesign that went into beta in early April, and is now the default homepage seems to be paying off with a 28 percent rise in unique visitors in April and more time spent on the site ComScore only estimates 188,000 unique visitors in that month (Quantcast puts it a bit higher at 241,000), so it is still extremely niche, but at least its numbers are now headed in the right direction. The redesign makes the whole feed update continuously now so you can really watch your what your friends are doing on the Web as they are doing it.

Complain all your like, but the stream is here to stay and it will only get stronger.