Apple is notorious for doing very few acquisitions, I fail to see why Twitter, a company that is struggling to monetize, would be the exception. What leg up does Twitter give them and does it have the potential to generate billions in revenue? I have my doubts. See Apple's half-hearted attempt with Ping to show where their priorities lie.
If Apple really wanted to do a social network, they could very easily have already, Twitter doesn't solve any problems for them.