Personally, I would simply not be interested in working for an app developer like Instagram. The only serious problem they faced is getting popular. Sure, there is scaling, but there are so many people that need to scale and many of them also have other interesting problems.

Further, I would not want to work for Facebook for any amount of money. I personally dont' value their service, even though many do, and would feel that I was wasting my short life time.

Finally, I would not have bet on them earning that much in a buyout and think that it's a result of a current bubble more than anything else. I suspect that looking back, their purchase will be a prime example of the 2010-201? bubble.