How often to you speak with your kids?

My kids all live in the UK. The eldest is disabled and doesn't speak, nor does she pay much attention to a computer screen or even recognise us.
The next one doesn't really 'do' internet and despite having an email address, a Facebook account and a mobile phone, he never uses them, so sending messages or ringing him, it could be months before he picks them up.
The middle child does respond to text and Facebook messages, but rarely makes contact with us outside of Christmas, Birthdays and Mother/Father's day.
The 2nd youngest is on twitter, Facebook, instagram and we chat daily, comment on each others pictures and updates and Skype often.
The youngest is in boarding school, Facebook etc is filtered through the school's wifi and 3G is almost none exsistant as the walls of the school are very thick and the location is remote, although I can see he's seen the whats app pictures and comments I send him, although he rarely replies. I do phone him on his mobile once a week but he only replies if he happens to be somewhere where there is reception and the call is usually cut off.

I send postcards but the boys always say 'don't bother Mum' or 'I'd rather you didn't as it gets read by everyone first'. I send letters with small gifts or money, but I rarely hear if they've received it.

I did get a post card from the middle child a few weeks ago, but only because he girl friend made him.

The thing is, it doesn't bother or upset me, the kids don't mind either, or so they say. I visit the UK twice a year, they come here once or twice a year and they have family in the UK, cousins, aunts, uncles etc that they spend most of their time with and the 2 eldest boys are in serious relationships so MUM is way down the pecking order anyway.