Post exchanges are so obvious and devalue sites, IMO.
There's nothing worse than looking at the 20 newest threads on a forum and they are all by the same member with titles such as:
What's your favourite cheese?
What's your favourite bread?
What's your favourite cheese and bread combination?
What's your least favourite cheese?
etc.
I join sites if I am interested in the subject matter, no other reason.
Or at least I used to; no time these days.