"We first had this debate when desktop internet access came in the '90s. My wise old boss, the infrastructure manager back then, held the opinion it was inherently self-limiting. Certainly people could spend all day on dodgy Geocities sites, but it would soon show up in their output. It was a line management issue, and the best managers manage their people by what the achieve, not by the hours they put in."
This. Some people now, rather than "gathering around the water cooler", they tweet and so on. If they do it too much, they will be unproductive. As an anecdote of when it's obvious someone's being unproductive, I was unloading some pallets of PCs with some student employees, which took maybe 10 minutes. One or two of the guys stop dead in their tracks like 2 or 3 minutes in, start reading and firing off messages the whole time, while everyone else is moving computers. Almost all the rest of them used their phones too, but after the unloading was done when there was nothing much to do for the next 10 or so minutes. There's an easy to follow etiquette, don't use the phone when you're in the middle of something, and use common sense and moderation.
Taking Victor Vinge's view on these things, I would say some of this usage makes the phone actually count as an external cybernetic enhancement. I've seen a few people that can actually text or whatever and hold a voice conversation at the same time; they are not alternating between speech and typing, they can speak articulately (not just a "Yeah.." or whatever) and type at full speed (at least full speed for a phone keyboard...) at the same time. Some of these people really do find it difficult to deal with being out of contact all day, it's like telling them they can't speak.