I have a theory that one of the things that makes twitter appealing to many people is it's ephemeral nature. Seen this way it's historical flakiness is actually one of the the things that makes it popular. Whether valid or not, there is a sense in which twitter seems to be a good venue for casual social messages that aren't going to be archived and potentially used against you at some later date. Whereas facebook (again validly or not) sends a message that this is the repository of your accomplishments, and that it will be your permanent record.

The thing is that as human beings we need both fireproof digital libraries that can keep our memories safe, and the electronic equivalent of the floating world where the record of what was said evaporates with the morning dew.