If you choose to stay with Facebook (and it is entirely your choice), there are various things you can do to mitigate the effect of the privacy erosion. One of these things is locking down your profile. We all have different tolerances and needs, so do what you need to do to make yourself comfortable.
I’m throwing out this excellent guide for those who are confuzzled on the whole locking down process. It shows you in a series of screenshots a good number of the steps you can take.
Note, however, that due to the very nature of these posts, the abovementioned guide may cease to be accurate at some arbitrary point in time determined only by Zuckerberg’s whim. It is also worth checking regularly that your choices have not been overridden.
If you have comprehensive howto links that might add to this, explain it in a different way that others may understand better, or cater to a different social networking service altogether, please add them in the comments.
Note: Please take care not to be too ‘splainy here; howtos of your own should be posted to your own blog and linked instead. Any blamey comments (if it starts out akin to “well, I just don’t…” or “what do you expect…”, you’re being blamey) will be bitbucketed. With prejudice.