It is interesting to note that on urbandictionary, you do not need to be logged in to upvote a definition. For example, if you visit and upvote the first word, it will record your vote. Even if you open an incognito window and try to upvote again, or use a different browser, the system still detects that you have already voted.
A similar situation occurs on Yelp when clicking on the useful/funny/cool buttons.
The question arises: how are these platforms able to detect multiple votes from the same user? Is it simply recording IP addresses and checking for duplicate votes? The process seems very quick, almost like a client-side JavaScript operation, but how can they tell if I have already voted in a new incognito session?
One observation is that there may be a GET request with a callback parameter value, perhaps something like jQuery17108145102619146468_1379449108386.
An edit to this theory suggests that indeed, the platforms must be logging IP addresses. A coworker confirmed this by visiting the link and being told he had already voted, despite never having visited the site before. Further investigation revealed that our external IPs were the same.