I think that going icon-alone is a good decision if it works. It's risky and has a high potential to fail, but if it is successful I think it makes the branding more recognizable and instills a more personal feel.
Getting an "iconic" logo is hard to do, so if you can do it, I would definitely take the risk.
I think they have the room to be able to take that chance, especially with the mark being used in their mobile properties and how far-reaching the network has become.
I do think though this is minor compared to the search graph itself that they announced today, as it being a very major feature they've considered to be worthy for their platform. Very interested to see how it really performs with the content that's contributed daily by hundreds of millions of people.
*graph search. You guys know what I meant.