In late 2004, Thiel made a $500,000 angel investment in Facebook for 10.2% of the company and joined Facebook's board.
In 2005, Peter Thiel created the Founders Fund, a fund for angel investments and venture capital investments. Other partners in the fund include Sean Parker, Ken Howery, and Luke Nosek.
In addition to Facebook, Thiel has made early-stage investments in numerous startups (personally or through his venture capital fund), including Booktrack, Slide, LinkedIn, Friendster, Rapleaf, Geni.com, Yammer, Yelp, Inc., Powerset, Practice Fusion, Vator, Palantir Technologies, IronPort, Votizen, and Quora. Slide, LinkedIn, Yelp, Geni.com, and Yammer were founded by Thiel's former colleagues at PayPal: Slide by Levchin, Linkedin by Reid Hoffman, Yelp by Jeremy Stoppelman, Geni.com, Yammer by David Sacks and Xero by Rod Drury. Fortune magazine reports that PayPal alumni have founded or invested in dozens of startups with an aggregate value of around $30bn. In Silicon Valley circles, Thiel is colloquially referred to as the "Don of the PayPal Mafia", as noted in the Fortune magazine article. Thiel's views on management are highly regarded, especially his famous observation that start-up success is highly correlated with low CEO pay.