Paul, Jason is right, because it happen to a company I was involved with a few years ago. Facebook was in it's early days (I am trying to be as vague here as possible because I do not want to give away who was involved), and they had a problem to solve that was getting worse as they grew quickly. This company had a solution - it had been developed by a small, but smart, team over a relatively short period of time (9-12 months) and was proven to work well with this companies customers.
Facebook called in a meeting with this company, and talked to them a lot about acquiring the company for Facebook stock. They offered the staff roles within Facebook, and the negotiations were well advanced. During due diligence, one of the tech guys sat down with one of the Facebook execs/engineers and described how the system worked, how some features were implemented and ideas they had for future features.
Not long after Facebook cut off all contact, ended up developing their own solution, ended up implementing a lot of these features, and ended up shafting this company. It is a shame that Facebook is so narrow in their thinking, because the execs at this company really could have helped Facebook avoid some of the troubles they had.