According to Comscore data, Facebook added 40 million members in February. Meanwhile, MySpace is half the size today where a year ago both were serving over 115 million people per month. Today, Facebook serves over 275 million monthly unique visitors. While Steve Rubel claims Twitter is peaking as it recently jumped to 4 million monthly visitors from 2.6 million in one month, I don't see it. Many corporations have recently included their Twitter handle in their press releases, and it's getting more business users. There are an estimated 520 Twitter applications. The only thing that would slow them down is persistent service outage or lack of a viable monetization plan. They recently raised $35 million at a $250 million valuation, not bad. The key difference I see is their phenomenal success with the Twitter API proliferation. They got this right from the get-go as opposed to the starts & stops of Facebook. Twitter has a live of its own and is rapidly reaching critical mass.