Comment by internetter

1. Voting rings are one of the easiest types of spam to detect. Of course, the bigger the service the better the bots, but that problem specifically is a later issue

2. Zero tolerance to bots

3. The service is not free

4. Individuals have very little impact on the rankings of other users, so you need to pay for a lot of bots

I believe that the true problem with bots on, say, twitter, is that they have perverse incentives to 'boost engagement' and whatever by allowing the bots to run rampant.