A few minutes a day doesn't kill anyone.
At my old job as manager for a dental lab, the policy was up to me, and I let it go.
Texting, FB, it was fine on one condition, it does not significantly slow you down.
If I asked you to get something ready to go and you were texting, didn't really care, I would assume that a person could multi-task or prioritize, if it wasn't ready in a few minutes but you were still texting, I'd go grab the cell phone and toss it into my drawer.
Here at my place now it's basically the same thing, as long as it doesn't make a noticeable effect on your output in a negative way, I don't care. I think for the most part it helps with the afternoon bog down, being able to take a couple otherwise idle minutes, sip on a coffee and see what your aunt bobby joe looks like riding a racing lawn mower on a mud track on FB.
If facebook doesn't bring in any dollars for the main business I would have it blocked and put restrictions on cell phone use as well... if your paying them to work you are paying them to work not do their own thing.....