Even though I don't like advertising generally, I'd much rather look at a well-crafted Nike / Apple creative than the weight loss / degree mill tribal fusionish junk they run now.
My guess is the ad system they have now is just to test the waters... get the team learning the ins and outs, improving the system's algorithms and reporting capabilities, etc. Running branded ads is much more of a significant move: they're clearly ads, and not so much some sort of pseudo-relevant text ad. In other words, branded ads are much harder (as a user) to ignore, and would lead to a stifle in growth -- the absolute last thing that Facebook wants.