What does a teenager do when s/he is standing in a group with several 'pals' who are all socializing with other not-actually-present people by poking their mobile phones?
I guess the only acceptable thing to do is retaliate - start prodding at your own smartphone. Fight rudeness with rudeness.
Otherwise you just have to stand around like a servant awaiting the Master's orders.
Supposing you were standing with friends and instead of talking you pulled-out a book and started reading?
Doesn't happen; but if it did, it would be regarded as rude.
So why isn't social networking in company regarded as rude?
What seems to have happened is that social networking inherited the etiquette of the telephone - which has always led to appalling acts of rudeness.
When the phone rang, people would break-off (almost)anything - any conversation, no matter how important or serious, and (almost) any human interaction - to rush and answer.
I can remember travelling for five hours, at his request, to meet-with and chat-to to a famous medical Professor. Yet after I arrived he spent almost the whole of our relatively brief time together (including while driving me - one handed - to the railway station in his car) talking on his mobile phone to people he presumably regarded as more important than me.
So people give a similar priority to social networking that they did to telephoning - despite that the excuse of expensiveness and urgency (which originally, I suppose, justified dropping everything for the telephone) no longer applies.
Consequently, rudeness begets rudeness.