I've never thought of having SMS notifications before but for SMS notifications you must keep in mind that some people have only limited numbers of messages they can receive before they have to pay more $ to their provider. So notifications such as content rated and likes would not be important to receive through SMS.
If I was a member of a site I would want to only be notified on important activities, such as new messages received which has some details of the contents of the message, new friend requests with some details of username/gender/age, comments on profile with some details of the comment, comments on activities you have commented on...
Hope this helps.