- Peter Thiel was crucial for Mark Zuckerberg (Facebook)
- Ray Chambers for Jack Dorsey (Twitter)
- Ken Allard and investors Ben Horowitz for Dennis Crowley (Foursquare)
- Paul Graham and Partovi Brothersfor Drew Houston and Arash Ferdowsi (Dropbox founders)
- Adam D'Angelo for Kevin Systrom (Instagram)

See more of who mentored whom at
http://www.businessinsider.com/meet-the-mentors-behind-the-visionaries-of-tech-2012-7?op=1

For most, curiosity, pizza, and college. I think more people found key people after they got going than inspiration before starting.

For many it was Bill "Coach" Campbell of Inuit: http://en.m.wikipedia.org/wiki/William_Campbell_(business_executive)

Also check out Ben Horowitz's "The Hard Thing About Hard Things" for more on Bill.