Something that I haven't seen discussed much is the possibility that when a startup (i.e. Whatsapp) achieves billion-ish scale adoption in a space that overlaps with an incumbent (Facebook), it has already destroyed some value (monetizable user attention-time) in the incumbent, so the incumbent has to obtain the upstart (pun-intended) in order to regain that lost value, and simultaneously grow into the areas that they don't overlap.

In that case, any premium paid that isn't reclaimed by future value created as a result of the acquisition is destroyed wealth. Unfortunately, without a crystal ball, it's hard to say what that premium is, if it exists at all.