Most of the people I talk to on Twitter don't have money to spare. They're the kind of people who have to toss up a fundraiser to make rent because they had a surprise medical bill or car repair.
Or they could offer little widgets you can buy and post on your profile page, like LiveJournal does (assuming it still does; it's been ages.)
Or a combination of the two, like DeviantArt.
There's plenty of options. Hell, sell Twitter plushies. That failwhale image was pretty cute.