There is some social psychology and operations research around this -- not necessarily as it relates to human/machine interactions, but certainly as it relates to such quasi-analogous situations as call centers, lines, speed vs. variability of service at McDonald's, and so forth. The premise being practiced by Facebook takes a lot of cues from behavioral science data that is generalizable to many different domains.

McDonald's, to give a famous example, proved analytically a few years back that -- beyond a certain threshold of commonly expected service times -- their customers would rather get semi-slow service on a consistent basis than highly variable service. In a perfect world, of course, average wait times are as short as possible in addition to a minimization of variance. But when you're at the level of acceptability, there are diminishing returns on speed increases and increasing returns on reduced variance.

Alas, my understanding of this point is entirely folkloric. Self-experimentation, and working on improving real interactive systems, agrees with it. Please update if you find anything more comprehensive.