Algorithmia:

Sometimes, users in social networks might reach out to their friends (or even create fake accounts) in order to “seed vote” their submissions. This is obviously a form of playing the system, and possible solutions for this problem depends on your data structure and how your users interact with it. Many platforms rely on IP addresses and browser agents among other things. Given that we do not have that data available, we had to look elsewhere.

One of our engineers came up with a very plausible solution to this problem: assign a collusion ratio to every possible combination of users and posts that will depend exclusively on voting activity. This ratio is used as a proxy for a collusion probability. The closer it is to 1.0, the more likely that the subject group of users are colluding.