I've lived through two acquisitions by the world-largest companies (and few more smaller ones). Here's my impression of what often happens in situations like this:
* There's a huge disconnect between the actual product and what was sold to the buyer. It could be that the product was a borderline fraud, or it could be that the product was actually much better quality than the expectation on the buyer's side, but the buyer isn't interested in most of the product.
* Fear spreads in the acquired company that their product will be discontinued or reshaped into something else.
* The pay is good, probably much better than before the company was bought.
* Many internal teams end up lacking real purpose and try to insert themselves into every new internal project only to obfuscate their irrelevance.
This leads to some pathological developments, where the teams previously working on acquired products start doing a lot of useless, for-show work. Internal initiatives sprout like mushrooms after a summer rain, but they are all plagued by very broad (and mostly irrelevant) team involvement, duplication of existing products / services, and fear of being discovered. And while there are plenty of such initiatives, their role is to be a superficial distraction. In reality, everyone is afraid to touch the old code or do any sensible integration because it could lead to blanket firing of a lot of people. The middle-management behavior becomes a sort of exchange of favors, where everyone is afraid that the other can blackmail them into losing their job, and so everyone is trying to be extra nice by offering a slice of a pie to another manager.
What this leads to, in reality, is insane inertia (often despite highly shortened software release cycles), astronomic amounts of unmitigated tech. debt, opacity in communication with management, persecution of those who genuinely want to improve the system.
Based on this, my prediction is that Github will not survive. Just like Skype didn't. Somehow or other, Microsoft will find a way to replace the product with... MS Outlook with a new skin.