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.
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.