Quite a few conferences now require that experiments involved human subjects be reviewed by an ethics committee of some kind before being carried out. Universities have such committees and they do oversee themselves pretty effectively.

So no, I do not think that Facebook overseeing its own researchers is so far fetched. The ethical review board would probably consist of a combination of lawyers, PR people, and people with research backgrounds. The lawyers and PR folks would not have seen this experiment as a "great opportunity for research that was previously impossible" so much as "lawsuit bait" and "PR nightmare."