I think this says more about the significant amount of control Facebook has with its messaging platforms and the implications of them losing that control for even a few hours, rather than who happens to benefit when Facebook's users are forced to find alternatives. What happens to Facebook can now have ramifications for the entire Web.
It's gotten to the point where Facebook's userbase can act as a massive DDoS network not out of malice, as what the term usually implies, but because of the sheer number of people in the billions who willingly chose to use Facebook.