Definate yes, be there friend so you can discretely watch their posts. I would also say that you should know their passwords so from time to time you can monitor their messages. If they refuse deny access to Facebook in your home, (externally you may have no control), which will devestate them and also block it on their phone.
You do NOT have to let your child know that you have checked their messages / wall. I keep it to myself unless I see something worrying.
As teens expect some mild sexual humour, occasional swearing, and generally acting like a prat.
Only flag it with them if it starts to get out of hand ie: porn videos, offensive posts, bullying etc.
Do not send them game / app requests and don't post on their wall or they will find a way of blocking you / unfriending you because they will feel embarrassed.
You will also be able to monitor any bullying of your child which is often done through social networking sites.
In general you should have control over your teen / sub - teens internet access and usage but still allow them to feel they are completely free to make their own choices on what they view.
There are some programs that will block pornagraphic / unsuitable sites if your child has their own PC, some of these can be found online.
However nothing can surpass the act of actually going into your childs room or passing the family PC and asking what they are doing, ( as well as checking the history of sites visited, (either drop down box or favourites / tools - history on the toolbar)
You should also be aware that nothing online is completely foolproof and there is a chance that your child will view sexual or violent sites at some point and you should equip them with the knowledge of how to deal with this if they come across it by accident and also that if they deliberately attempt to view that this is unacceptable, ( propably won't work as well with teenage boys).
Other than that the internet is a great form of communication / information for teens - and it saves on your phone bill.
If you are not computer savvy then your local college and even high school may put on free classes so you know the basics and can keep your kids as safe as possible online.