The main reason is everyone else sold or overreacted to the market and pivoted.

Yammer sold for $1B+ to Microsoft but could've easily been a 30B+ market cap company.

Slide had some issues and continue to react to the market trying different things - had they focused on a real problem, they might've created a Zynga type company.

The rest of them (Dave @ 500 Startups, Jeremy at Yelp, etc) are still at it and the verdict isn't in.

That being said, the fact that 1 company has an alumni with such success - and There's many I haven't mentioned - is only testament to the quality of the people.

P.S. What's the market cap for Tesla? Elon is just getting started - I think I'll be WAY bigger than LinkedIn.