I have a question about craigslist, prostitution and law enforcement police?

So I noticed all these ads of people claiming to be generous or seeking someone to be generous... sounds a bit like prostitution. But can cops really go down on someone's house and bust their door for a craigslist post? I mean isn't there "innocent until proven guilty" like they have to SEE you soliciting a prostitute or simply catch you in the act? A simple post can't really get you arrested can it? Is that why it is so hard to catch these people?