Not everyone has a linkedin/facebook/twitter account.
True, but most people that would be hired by the startups listed would. No?
Most != everyone
Seriously, requiring sign up via accounts on sites like facebook, linkedin etc is a really stupid idea. Startups (I'm looking at you quora!) need to stop this bullshit.
Edit: Looks like quora now allows email sign ups. Good.
Many people, like myself, like to separate social media from the real world in some way, and this is one of those places I like to keep them separate.