a website where people share their websites and send the pages they visited to each other

A tool for Mozilla Firefox that is used for trolling at the speed of light.

The most addicting website ever.

stumble across, to encounter or discover (someone or something) by accident

(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.