>Modern hashes like SHA256 are probably resistant to collisions in Twitter’s limited message space. Actually, no. 140 characters alphanumeric translates to 833 bits of entropy. This is significantly more than the 256 bits in SHA256, and due to the pigeonhole principle, collisions are guaranteed. That said, it's still nontrivial to bruteforce the hash, because searching thorough 256 bits is hard, but that's unrelated…
Collisions are guaranteed, yes, but, you also have to keep in mind that the only collisions that matter are the interesting ones. That is, collisions between two different messages written in different languages, and where both messages make some sort of sense. Although I don't have a mathematical proof of it, I'd have to wager that those types of collisions are exceedingly rare.
Edit: At least "exceedingly rare within the space of plain-text messages smaller than N characters," for some reasonable value of N.