Tumblr is a lot closer to a social network than a forum, and I'd say only a small subset of it is comedy. Also, Tumblr has 460 million registered accounts and SA has 200k. That's probably partly because it costs $10 to sign up on SA, so alt accounts are uncommon, whereas accounts are cost-free on Tumblr, so some people may have many alt accounts. But Tumblr still gets way more traffic.