I think offline etiquette applies. It's "legitimate" that someone might want to know if site members actually earn anything on the site. Asking for numbers is out of line, as far as I'm concerned. A lot of people feel very free to share what they earn, so nobody even has to ask them. Besides preferring to keep this particular piece of information to themselves, a lot of people have concerns about others' sorting out/figuring out who makes what - and then proceeding to steal their content, ideas, and whatever else. So, to me, there's that extra element to online writing that makes asking someone what he earns even more out of line than offline.
Sometimes, too, a person can make x amount on a site like this one, but then make additional "x amounts" from other places online; so who makes what on here can be meaningless at times. Several people who do quite well on this site make no secret of their good earnings. THEN, though, there'll be other people who think they're lying about good earnings anyway. So, either way, exact figures shouldn't be needed by the person who wants a rough idea about whether anyone makes any money on a site; and if someone wants/need exact figures he ought to hold off until someone offers them - not ask.