Sadly, I'm on several social networking systems (namely Tribe, Facebook, MySpace, and Livejournal), and all of them seem to lack features that I keep thinking would be perfect for these networks. In no particular order, here are my thoughts-
1)Let's be fair. There are some parts of our lives that we would want to very much keep a secret from other people in our lives. Our employer doesn't need to know that we are a fan of Green Acres, nor do our parents need to know that we like women dipped in honey and chocolate. Nor do our friends need to read about the frustration we have with them (I have a friend that I love deeply, but hell is she bad on my ego when I play world-saver for her). The concept that I have is what I call the "hive"-six levels of control positive and negative over people you know.
You organize the people on the social network by one of three types-friends, family, and business. Postings can then be segregated on the various groups by type (a dating board, for example, would only let people that are posted as friends and not business). In addition, if you were to post a blog on the social networking site, you could restrict it by type...and by levels of friendship.
Remember I said about six levels of control? You could set the levels on each person that is a friend on your list by 1 ("kinda know them") to 6 ("best friends forever/lovers"). Photos would be controled by levels-from open to the public (0) to the photos you only want some people to see (3) to photos you maybe want one or two people to see (6). Your blog postings would be controlled like this-"only friends 3 and above", "only business three and five", etc, etc. On your friend's profile, you could see friends of your friends...and how they know them, and how their levels are in comparison to yours-a good tool to sort out people you know and don't know.
Of course, we have the reverse controls-the ability to list somebody from -1 (not a fan of them) to -6 (don't want them to know ANYTHING of what I do here). If somebody has a lot of -1s and belows, maybe you shouldn't know them...or let them find things out.
2)At the end, the purpose of social networking sites is user-created content. Groups for things you didn't think there was a need for (kinky knitters of making toys out of yarn, I swear to God they exist), organizing events of all types, finding things to buy and parties to go to. Group control should let you be able to set up everything from parties to sales....of course there will be open, CL-type boards, but the ability to put add-on modules such as event calenders and photo galleries and similar types of hardware.
3)For handling adult content....I'm thinking of a three-tier system. The first tier is the "free" tier-kids and such can reach this level. This keeps them out of content specially designated as "adult" (and, conversely, there are two tier levels, 18+ Adult and 21+ Adult). I'm still trying to figure out a way to keep kids away from annoying people...but the idea hasn't hit me yet.
Second tier is "adult"-and if you want to get to the adult boards, you have to pay...$1 a year. You have to have a credit card that is linked to your name and your address (to prevent fraud), and that gives you access to the Adult boards.
Third tier is "patron"-for a monthly or yearly fee, you don't get any ads, any postings get put on the top of the screen and any friend listings when you're online, etc, etc...
There are other ideas, but they're all being worked through my brain...very, painfully slowly.