- 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
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.