Mxit is just a form of communication, and you should be able to trust your children on it, you should not treat them like prisoners, if you fell your child at 16 should have a certain time he needs to go to bed, then that's fine, personally, I feel it's kinda strange at that age...I fell asleep on mxit often when I was still in my teens, and it never affected me...
the probelm with mxit is that your child(daughter) could be chatting to some weirdo and fall in love with him over the phone, and would want to meet him, or send him pics...inappropriate pics....but if you teach your child the dangers of mxit, and the dangers of chatting to strangers...you'll leave them with guidance and trust...that should be enough...if you treat your teen like a toddler...you won't achieve anything...children who aren't allowed to do certain things will do them behind your back, I'm all for rules, but don't over power them, don't strangle them...most teen girls ,who's parents won't allow them to go out with their friends(clubbing or whatever) , who's parents just come across to up tight, (that the teens aren't brave enough to ask , scared of the reaction they might get) or those who believe their 16yr old daughter is still 12 and treats her that way, will at the first opportunity lie about their plans, go and "sleep at a friend's house" the friend will probably spin her parents the same story...and they'll end up going out anyway, they'll get drunk, knowing they won't have to come home, some guy will probably take advantage of them, they'll wake up in some older guy's bed...and you'll be the only one to blame...
all situations can be managed, if you personally pick up your child when they go out, when you give them a curfew, and make sure you know the people they go out with...every situation can be managed