They could offer "premium" accounts, the way Amazon does or Hulu used to, where there are no ads and some add-in services are discounted or free.

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.