The only difference is that a post on Twitter has artificially limited size. This is a restriction imposed by Twitter and as such could be handled by them - for example by showing a short summary + a link to full message for posts that were imported from other networks.
Maybe you could. Twitter and Tumblr seem to be about the same modulo the length. Isn't the advantage of Twitter its shortness? Maybe we'd discover what the advantage of Twitter is.
And perhaps Facebook is the same just with better networking features. Maybe we should try to write federated Facebook and Twitter clones to prove it can be done.