Here's the case for GitHub: Facebook, as mentioned in parent comments, has essentially tapped out casual social networking as a growth driver. But casual (e.g

Here's the case for GitHub: Facebook, as mentioned in parent comments, has essentially tapped out casual social networking as a growth driver. But casual (e.g. your mom and uncle) is the key there. There are many other kinds of social networking: not as large, but large enough to be worth even Facebook's time. Developer networking is absolutely a large and valuable demographic. Facebook has demonstrated competency is compiling and making social graphs useful. Ergo, buying GitHub would allow Facebook to move into a new market while offering new features that GitHub couldn't build on their own. (Note: I'm charting out an "If Facebook decided not to be stupid and just slap ads on GitHub" business case here)