Image via WikipediaApple and Microsoft were born around the same time. They were not at peace. Netscape came along. Microsoft killed Netscape. 
Google offered to sell itself to 
Yahoo. Yahoo refused. A few years later 
Bill Gates offered to buy Google "at any price." Google refused. Google tried to buy or bury 
Facebook. Facebook survived. Facebook tried to buy 
Twitter. Twitter refused. So Facebook hunkered down and "learned" as much as possible from Twitter. Facebook has tried to buy 
FourSquare, more recently it has tried to bury it. 
See, there is that buzz factor. The company that had the crown seat in the buzz kingdom until recently is able to spot the next taker and gets uncomfortable.