Twitter will never be the new facebook for a very simple reason: Facebook is conceived so that people that have nothing of interesting to say (a huge percentage, sadly) can still tell a lot of useless things, post photos, take a quiz, comment some stupid thing about the status of somebody else.
Twitter instead is designed for people that have something to say (otherwise no-one will follow you given that there is no the reciprocal bit) and are even able to say it in 140 chars.
As a twitter user I'm very glad of this, since I hope that staying an elite thing twitter will continue to be interesting and worth to use. My claim is that also twitter owners should be happy about this. In the long run it can be more interesting to have 1% of the users but with good content than 99% of users that don't pay a dollar and produce things that are not of interest.