Urban Dictionary is bigger (30MM page views/month[2]), but Pulse is an order of magnitude bigger ("100Ms of requests per day" [3]). Pulse ships on a lot of Android devices by default now.

[1] http://ban.jo/blog/banjo-hits-500k-users-in-under-six-months

[2] http://success.heroku.com/urbandictionary

[3] http://googleappengine.blogspot.com/2011/11/scaling-with-kin...