(N) 1. Stumbleupon is an internet sea of feminine passive-aggression, trolling and insanity. An ideal forum for internet tough guys, pretentious bitches, would be political pundits, old women and shitty poets to sound off. Members of the 'core forums' aren't well known for having much going on with respect to interpersonal skills, as is evidenced by their endless hours spent online.
2. A place for douche bags too old for 4chan to hang out.
3. A web browser utility used for trading kittens photographs, smut, and astronomical photos amongst 'friends', who are generally people you wouldn't spend a minute with in real life.
I joined Stumbleupon, but hated the experience. It's like facebook for wordy douche bags.