I see that we went in a huge machlukas haposkim in regards if keeping this site open for posts on shabbas involves a shaila of lifnei eever. I see that B"H we have rabanim and dayanim here. But this issue has been discussed and written a lot by famous rabanim, (see ohr yisroel volume 39 - about 40 pages about this, with like 15 rabanim writing about this), and they have come to a conclusion that מעיקר הדין there is no איסור involved in this:
1. a lot of poskim agree that there is no lifnei eever doing a איסור דרבנן
2. there is a thing called: "תרי עברי דנהרא" meaning that if a person would do this anyway, you are not helping him out, and these people would post on FT etc instead, so there is no איסור.
3. since this is not a thing that we give him directly, it's his own choice, there is no איסור involved.
See there for a lot of different היתרים.
But, I do have one big question about keeping this whole site in shabbas, where if somebody would use one of the links to apply for a credit card (for example) and give the owners a referral payment, that should be considered שכר שבת. I have spoken about this with a few rabanim, and nobody got on a real היתר.