I like to say StumbleUpon provides a personal tour of the Internet. The responses are more targeted to your interests than they would be with a regular search engine. If you choose a topic on our site that you're interested in, such as art, Web sites related to art appear, as if you're leafing through an art magazine.
If the founders of a start-up are considering selling it, I'd advise them to consider the synergies. Could the buyer give you access to something you don't have now, like a certain technology? Would it make your life easier? Are you looking for a change? Things will change, so you have to be ready for that.
The way the Facebook network is set up, it's not as suitable for content discovery. Twitter is better, but there are too many over-sharers. Also, on Twitter and Facebook, everything comes from people you know. On StumbleUpon, it comes from people that you don't necessarily know but share your interests.