That reduces your anonymity almost to the point of being useless. It means that any middle node connecting to a "Twitter" exit node would know you're almost certainly connecting to Twitter (effectively making the three-hop Tor circuit a two-hop circuit). In addition, Tor uses long-term guard nodes (the first hop) in order to defend against certain attacks. This means that the middle node could make a guess about what bucket a user is in based on the guard node. It's not as bad as not using Tor at all, but it gives middle relay nodes far more information than they normally would have -- and middle relay nodes are the easiest kind of node to set up.

The main reason why Tor has strong privacy guarantees is because every piece of traffic looks like every other piece of traffic. Changing what paths you pick based on where you're going undermines that in a fundamental way.