In the Further Education college I teach in, 98% of students have a mobile phone. Many of those are basic pay as you go dumb phones. Some feature phones, Blackberry phones are popular in UK with teenagers. A few have iPhones and Samsung and the LG slabby things. Not all have Web access on the phone (or have it but don't want to turn it on).
A result that surprised me was that teenagers don't access the Web that much on laptops or desktops.
The common denominator here is sms. I get 98% access. It costs a little money, but not huge. I hope it does not go away as we have brand (phone, carrier) dependent messaging.