I found this in a blog note. No idea if its any trustful though:
Graph Search A/B Testing
Facebook is doing some A/B testing on their Graph Search.
They are testing their Original layout, vs 2 new layouts (A & B).
A few weeks from now, we should be able to see the declared winner (if
Apparently Facebook is testing your hypothesis: Making search field look more like search field helps. Help what, that I do not know.
An interview with Rasmussen (Graph Search PO) tends to confirm it is A/B testing:
Facebook Graph Search has yet to roll out to all users, and Rasmussen
writes that part of the reason for the partial rollout so far was to
allow live A/B testing on real users — a process that circumvents the
possibility of endless internal discussions. “Without live usage we’d
just be arguing all day,” he writes.