Facebook turned down Yahoo's $1bn offer because Mark Zuckerberg realised that Yahoo didn't really understand social, and that they were making an offer because 'social is the hottest thing right now'. It's entirely understandable that a founder wouldn't want to sell their business to someone who wouldn't know what to do with it.

A Facebook competitor faced with an offer from Facebook would not have that problem. It's very likely that the competitor and Facebook would be very well aligned strategically. Selling would mean the competitor would get to where they want to go. Very few founders would say capable of saying no.