I honestly don't think it's the big fall of society that the luddite brigade likes to make out.
If people have something they are genuinely interested in talking about and a few things genuinely in common, conversation will happen naturally and no one will even think about their phones much in the first place.
If however people are:
* Visiting family etc out of a sense of duty
* Hanging out with someone they have nothing in common with out of a sense of habit / loyalty
* Hanging out with someone they do have things in common with but have talked everything to death already
* Generally not feeling talkative
* Introverted or just generally quiet people
And probably many other things, then it just relaxes people and fills a gap where you'd otherwise have awkward silences and staring. We seem to have built up this idea that we HAVE to be keeping up a constant, unending conversation that goes on and on ranging from topic to topic for the entire duration that we're around other people, and can't just enjoy each others company in moments of silence.
Doesn't mean you're a boring person or anyone thinks less of your conversational abilities.
Or somewhere in the middle you might have someone like me who is fairly shy and nervous and flicking back and forth through phone screens but still interested in the conversation... though you can tell when I'm engrossed in the phone vs. when it's aimlessly flicking and scrolling...
Give me a moment of catching up on the smartphone over "so........... uh..... how's work?" ANY day, and the beauty of it is you can find something interesting on it and share it and chat about that! So they can definitely be a tool for good, if used in that way.
Now, if I know that someone finds mobile phones offensive in social situations then it stays in the pocket and I'll um and ahh and ask about work, talk about the weather and study what's written on my beer bottle. In the unlikely event I was on a date or something, same thing. But as a general default state I don't see them as a massive evil.