I noticed a similar thing happens with their regional offices, while Google and other companies show their branding outside the buildings, in buildings with FB offices most of the times you don't know there is one just by looking outside or even at the building reception. At least this was the perception I had from the ones I have seen.
As a current FB employee this matches what I've seen of the offices I've visited. In fact, all swag I've received in the year I've been here also doesn't have FB branding on the exterior.
Regarding the original article, I've received no warnings about wearing FB branded stuff outside, so the warning may have been specific to DC or specific locations.
