Babak Nivi Co-Founder AngelList and Venture Hacks and angel investor
“Social proof is when you do something because other people are doing it. 
In this context, social proof is looking at what other investors, entrepreneurs, and advisors are doing. Customers don't count.
I'll disagree with Naval Ravikant [Co-Founder AngelList and Venture Hacks] on "An investor is choosing not to invest in your company and is making the introduction for you". I don't think this is as bad as he makes it out to be.
 [The] logical part of your brain will ask why the investor is passing. And there's often a good reason: wrong market, wrong stage, wrong geography, bad chemistry with entrepreneur, some known risks that the investor doesn't want to take, etc.
Rejecting a deal just because a peer rejected it is mathematically unsound. Here's the proof. Every deal you ever invested in was probably rejected by a peer at around the same time you made the investment. So, if your criterion is that you will reject deals that peers reject, you will never make any investments.” Babak Nivi, What is good social proof? Aug. 20, 2010; http://www.quora.com/What-is-good-social-proof