However, when I only have a sentence in my head, it seems silly to write a one-sentence post, and so I write it on Facebook.
That, combined with the fact that writing a couple of sentences on the phone is about all I can be bothered with (and the phone is what I tend to have in my hand when I'm bored between places) means that Facebook gets all the love.
And so I am going to try an experiment. The next few times a sentence pops into my head I shall open up Semagic (or the update page) and write it down there. And then I shall see if it sprouts into a post, or remains a single sentence.