Effective way to deal with misuse of mobile devices in class
There is nothing new in what you describe. As a student who oft got bored during lectures, I have done some distracting things during them, even though I didn't have access to any electronic device at the time (I'm young enough that they existed, but it wasn't accepted practice to bring them to class). We did crossword puzzles (collaboratively), we played cadavre exquis, we read newspapers and discussed them, we flirted, we worked on other stuff, … What makes you think the situation you are experiencing is specific to mobile devices? The cause of the distractions is different from other situations, but I think the issue is the same and the solutions are, too.

So, what I would suggest is: warn your students that while what they do is their business, you expect that their behavior does not interfere with other's study or the general class atmosphere. Tell them that mobile devices are useful, but that they should be careful not to abuse them. Handle non-compliance as you would a typical incident: talk to them, give fair warning, but clearly set the limits and enforce them if it comes to that.