I imagine the folks at Wikipedia had a few incidents of people being helped with their depression and killing themselves subsequently (possibly as a result of the help they received), then Wikipedia was like "What should we do? How about a don't-talk-with-suicidal-people policy?" without looking at the other, much more obscure cases where someone asked for help and did not subsequently commit suicide (possibly as a result of the help they received).

I don't know. Wikipedia spends a lot less time on socializing then LW does.

ETA: gwern, what's your take on http://www.reddit.com/r/suicidewatch?

No one can stop them even if it's a terrible idea, they have nothing to lose, and nevertheless, they list their list of suicide hotlines prominently in the sidebar.