Facebook co-founder and CTO Adam D’Angelo is leaving Facebook, according to multiple sources.

The reasons behind D’Angelo’s departure have so far not been disclosed, but it’s a strange move given D’Angelo’s supposed close relationship with Mark Zuckerberg; D’Angelo has been often cited as Zuckerberg’s best friend. It’s also a rare employee move away from Facebook at a time the social networking giant is attracting the cream of Valley talent.

As CTO at Facebook, D’Angelo oversaw the Platform Development team, the Data team and new product design and architecture. As well as holding a bachelor’s degree in computer science from Caltech, D’Angelo was named one of the top 24 finalists in the international Topcoder Collegiate Challenge in 2005, which competition which tests the ability to design and implement complex algorithms in a timed environment.

Could there be trouble at Facebook, or is D’Angelo simply looking for a new challenge, safe in the knowledge that he holds millions in Facebook stock? Time will tell: no doubt though that with those credentials he’ll pop up somewhere soon.

(via VentureBeat)

Update:Kara Swisher reports rumors of “tension with Zuckerberg” as a reason, but the official line is D’Angelo wants to take some “time off”. Another of Kara’s sources claims “D’Angelo felt his responsibilities were no longer fit well with his skills and interests.”