I don't contribute myself, because in order to do that, I would have to feel like I know enough about the subject - "enough" being "more than the other people who are here". When I stumble upon a wiki, it's usually because I'm looking for something, and I have a question - not the answer.
I used the Mass Effect wiki for both Mass Effect and Mass Effect 2, and it helped immensely (especially the planet maps that soothed the agony of using the Mako in the first game). I always end up reading more than I planned... and sometimes more than I should, because I tend to spoil the game for myself. Urgh, very bad habit right there.