Given two possible FB usage trends [1], (1) high levels of usage leading to a quick burn out and "tuning out" from FB, or (2) moderate usage of FB over a very long timespan (possibly even a lifetime), it's pretty clear that (2) is preferable for a company that has become public and must become sustainable over a timespan of decades to continue its survival. Option (1) would be ideal for a startup looking for a quick flip, but FB is obviously past that stage. Their preference for avoiding user burnout is hardly surprising [2].
Offtopic, but I'm rather dismayed by seeing a meme on a "news" site :(.
[1] Clearly these are not the only two options in the universe, but they're the most illustrative for this purpose.
[2] Though I suppose that a public admission of this fact is somewhat noteworthy.