There is something exciting about receiving an alert on my phone. It's just too enticing not to check. I've since turned off audible alerts for my social media apps as those are not something that I feel need to interrupt what I am currently doing. I only have alerts for direct messages like email, SMS, and other messaging apps.
I still socialise as much as I did before phones. It's just that with the information age and internet, I've been able to converse with more people who share common interests with me. I must say that there are many topics that do no interest people I actually know and see on a daily or weekly basis. My phone and PC enables me to connect with them.
With so many more people that everyone can connect with on their phone, there is more demand for people's attention. Before, when say sitting at the dinner table, you only interacted with others around the same table. If the conversation becomes stale for you (and it does happen), all you could do before was just sit there bored. Now, you can check your phone to do something about that boredom. Yeah, many people consider it rude. Certainly social protocol and etiquette demand you do not turn your attention away. Perhaps changing the subject may help in such situations.
I must admit that all this technology offers so many distractions in almost any situation. As long as I am engaged fully with the people I am in the company of, I won't be looking to act upon any potential distraction. However, a break in the conversation or whatever social activity I am enaged in allows for a quick check of my phone. It's almost instinctive now. I see friends do the same as well. It seems that this behaviour amongst my friends and family with smartphones do not cause any stir and is becoming a social norm amongst this group of friends.
I certainly don't mind my friends checking their phones occasionally. It seems that the people who do complain about this phone checking behaviour are those who don't have a smartphone. Times are changing and social behaviour changes as well. Before getting my smartphone, I would also consider this behaviour rude. I can understand where people feel such behaviour is rude. I've since moved with the times as well as many of my friends with smartphones. The times they are a changing and will continue as to change as technology advance.