I bet at least 50% of my work friends check Facebook every single day. For those under age 30 it's probably closer to 100%.
Twitter isn't nearly as popular, as far as I can tell; maybe 5% of my colleagues actively post messages under their real names. It's hard to say how many people check their Twitter streams regularly. I'm sure it's higher, but not that much higher.
Contrast that with China, where Weibo is absolutely dominating. Here among my work colleagues I'd guess maybe 25% are active users -- a couple times a day -- under their own names, and probably 2/3rds are active under pseudonyms (though for obvious reasons it's hard to tell for sure).
Why is Weibo better than Twitter? Because it's the first social networking system I've seen that adds a competitive element to status updates. On Twitter, some people obsess about their number of followers; on Foursquare people obsess about mayorships. But on Weibo, there's an entire scoring system based on how often you post -- and critically -- how well your posts are received. The result is that people are incented to produce better and better content, which results in more readership, which drives more reasons to make content.
Like Twitter (or Facebook), Weibo has the concept of posting links to news items. But thanks to the incentive system, 60% of Chinese microbloggers say Weibo is their main source of news (versus only 9% for Facebook or Twitter-using Americans).
If I can find a good way to post automatically from Weibo to Twitter to Facebook, I'm switching, and I bet you'll want to switch too.