If they're making decent money out of it, then I'd probably get money out of it, one way or another.

If they aren't, and they give me recognition, then I have no reason to have any problem unless it's in some way embarrassing, in which case I'd ask them politely to take it down.

EDIT: If it included a secret of some sort then I'd demand they take it down straight away, though, and they'd lose a chunk of my trust.

EDIT: And if they don't give recognition then I'd request they do so. If they claim it's theirs, I'd make it known to some kind of authority that they are saying my work is their own, and request that they not simply remove it, but make some kind of note in the thread (or whatever else it was published in) stating that the works were in fact mine, as I wouldn't want people to be under the impression that my works belonged to anyone but myself.