Paul Buchheit is the man known for creating the first version of Gmail.
He later co-founded FriendFeed which was later acquired by Facebook.
He has been working for Facebook all this time and might have been involved in the development of the much talked about Facebook Mail service.
He is now leaving the company and is joining Y Combinator as a general partner.
This is one of the rare cases where Facebook is losing a top engineer. They have been attracting a lot of talent from other companies especially Google.
Facebook recently hired Lars Rasmussen from Google. He was involved in developing Google Maps and Google Wave.
Considering Google Wave is very similar in nature to what Facebook Mail might be all about, Facebook is not going to miss much with this new development.