Not to alarm you, but actually I think a company would be entitled to use your tweet in their ads (with or without involvement by twitter).

A tweet is meant for public consumption (if it's public), you mention their product...as long as they don't make it look like you're saying something you're not, saying that you said that seems to be just stating a fact (which is well covered by the first amendment), and as long as they've only taken a sentence or a fragment of a sentence, they should be well covered by fair use as far as copyright is concerned.

I'm having trouble finding case law on this, so someone please correct me if I'm wrong.