Anyone with a web presence can probably tell you that Twitter's growing extremely fast.  It's outpacing Facebook's growth in the same period, according to Nick O'Neill's analysis using Google Trends.  Here's the chart:

However, I am not convinced that this is a fair comparison.

• First of all, the early Facebook was not an open service. It required you to possess a dot-edu email address to register.  The comparison should be made to Twitter's college student user base.
• Second, the price of admission to Facebook has always been considerable higher than Twitter.  A FB account without a full profile is less meaningful and severely limits the user experience.  Part of the cost of being a Facebook user is the profile completion effort.  Twitter does not have this barrier.
• And finally, I attribute a part of Twitter's growth pace to the existing proliferation of social networks, including Facebook.  Facebook has been a catalyzer to Twitter.

 It's tempting to compare Twitter and Facebook.  However, I think it is as misplaced a comparison as the common Facebook / MySpace deathmatch.  Twitter is not a technology company.  It's a great feature, and I don't mean that dismissively, as I think you can build a great company with a feature.  Nevertheless, I'd be worried, if I ran Twitter, of its long term viability.  There will be only one owner of the social graph and the current favorite to win is Facebook.