Not sure about other countries. But for india, its mostly useless. The sms is delivered depends on the network. And indian mobile network is always congested with so many SMS cos sms is cheaper than calls. So why facebook mobile is useless ? When someone sends you a friend request then it will reach you after 3 to 4 days.
Before few months, i got these sms delivered instantly. But now it is way too slow. I have added my friends request about a week ago. And today i got the add friend request sms.
I am sure facebook must tweak the mobile server synchronized with the facebook activities. For eg: If i have added my friend already then they should delete the sms queue from the mobile server.
This way they can improve the efficiency of the Mobile server and the facebook mobile application.