Reddit is somewhat susceptible. I have no inside track on my info, but the admins seem to have built some sneaky mechanisms for dealing with problematic users. My favorite is the practice of "ghosting" accounts, where it looks to the account user like their comments, upvotes, etc. are being counted when in reality they are invisible to the rest of the community. It seems to work.
Of course, for all Reddit's antipathy toward Digg, they had the advantage of learning from Digg's mistakes.