I can't help myself, but Twitter seems to me like a subset of Facebook. Something like ICQ, AIM, or Yahoo Messenger is to Facebook Chat or Skype.<p>It came at the right time in the US, but it takes some time for these things to get abroad. While it arrived to Central Europe, Facebook had enough users and introduced features like Followers etc. Today, Twitter is solely used by a marginal tech/hip/geek community here.<p>And advertising goes where people go. The question is, whether this evidence suggests that the Twitter of today can't compete directly with Facebook, and it's success is based on acquiring users before FB matched it in functionality. How stable this user base is? Is it a similar case as with the chat applications?<p>Anecdotal evidence - a friend of mine just asked, whether one should invest in Twitter... on Facebook.