Posting something on Facebook/Twitter is like standing in front of all your "friends" with a megaphone and shouting things at them. You can't pick and choose who hears it. You can try to get around that - selectively allowing followers, making multiple accounts, private feeds, etc. - but ultimately you have to self-censor what you put online.
I'll freely admit I sometimes post things on Twitter that I wouldn't if my mother were on it (and following me) - she doesn't need to know everything. But I also opt to not mention when I've been invited to a party, when I'm going on vacation, when I've got a semi-private event going on, etc. because I have overlapping groups of friends and I can usually think of at least one person who would get their feelings hurt if they realized I was doing X and didn't invite them along. Some things really are still better done in person.