The fact that twitter has a size limit of 280 bytes (used to be 140 bytes), they are probably trying to free up some inodes to save some disk space, as greedy as it sounds.
Storing each tweet as one file might even make sense. But you need to pick a filesystem that supports that use case at scale, if you are twitter.
https://en.wikipedia.org/wiki/Maildir is a good example of how this can make sense---at least at a small local scale.