Maybe I'm weird, but I believe in the theory that (all else equal) it's good for business to minimize how much your users hate your product/service.

In other words, users dislike the feeling of not knowing whether things are ads. I can't see any real downside to labeling them, so you're better off doing it so you don't drive users away.

There's also the matter of brand reputation. You don't want to make your ads seem like your own or your user's public communications.