While, yes, most everything you post online remains online for quite some time, and there's not really a whole lot you can do about that.
There are however quite simple means to deal with that -- means you use offline all the time without thinking about it: Social circles. You most likely wouldn't dream of bringing your boss or parents out drinking with your drinking buddies? That's because they're from different social circles, and meet different identities (your professional identity, and your casual friends-and-family identity respectively). And such social circles are typically quite stratified.
There is nothing keeping you from arranging similar systems online. Nom de plumes and aliases have been used for ages in the analog world, and with good reason. While using different account for different social circles is no guarantee information won't seep between them (same as rumors do in the analog world), it will at least limit this information bleed.
Actually realizing this is quite easy depending on which networks you're using, most of them send you an email when something interesting has happened, so you really don't have to constantly check every single account you have (which, even if it's just two per social network site, can reach quite large numbers.)
Though there are actual account-switching issues as, I think, you're not expected to have more than one online identity: Gmail for example has a quite frustrating bug that sometimes occurs when you log out and forget to close every single google-related browser tab you have open before logging into another account, where it breaks the session and you have to go to the google main page and log out from there.
Another option would of course be using different social networking sites for different social circles, but that has it's share of issues as well, including being met by anger and frustration when you refuse to add someone you know well in real life as a contact on the "wrong" site.