Well, there are websites that attract millions of people to them-here are some.
it is the WORST rpg game...but it attracts some serious crowds, all RPG guys and gals, so it's a good spot to advertise
again, millions of people-one of them is bound to want to play.
A browser based strategy game-I play it and I found a different space adventure, that I liked. It has a couple hundred people among its ranks, possibly on the brink of a thousand.
This is a two dimensional space adventure. You can get to lvl 20 without pay, then you pay a monthly fee-I'm sure someone would prefer a one pay deal-and three dimensions. This site drws thousands.
This site draws well over one million people every day. It isn't really a all sci-fi community, but it gets enough people so that the odds are one of them will play.
Thousands of players, all browser based mmorpg egg-heads. It is a strategy turn based sorta game, but thare aren't any banners yet, I don't think, so it may not be possible-or you will be the only banner up there!
That's it. All the sites I can think of. Hope you can attract some players.