That said equating what someone posts on this forum vs. Facebook is quite a different argument... sure if someone (say "Big Brother") is interested, they may have the resources to know about it, but this is far cry from Facebook who evaluates/relates all the data provided. Mapping your profile to other users, using facial recognition software to identify you in every photo posted, grabbing your GPS data from your phone, uploading all of your smartphones contacts to their servers, or changing your profiles e-mail address without permission... it never really ends, and no matter what you do, any data provided to them will never be deleted.
But its like anything else... some people are comfortable with using G-mail as their email provider too (who's user agreement allows them full access to read your e-mail). Could other providers be reading your e-mail? Possibly... but you probably didn't explicitly sign that away at any rate.
![Wink ;-)](./images/smilies/icon_wink.gif)
Most everything on the internet is based on some form of trust (even/especially at the core infrastructure level... read up on how the Chinese "accidentally" routed U.S. internet traffic to servers in that country)... how much and who you give that trust to is up to you.