If a large number of people don't know what they're doing, that's a UI design fail. It doesn't matter whether they should have known.
(I suspect Twitter has the same problem. People should know it's completely public, but I suspect many don't.)
Even though im a programmer, the only thing i do in facebook is add friends, it already affected me to just have an account there.
The effect was neutral to good..this time.
Edited as follows:
[M]y realtor checked my background with Facebook. I only knew about this because [he] turned out to have gone to high school with my boss, and my boss told me.
[T]he only thing i do in [F]acebook is add friends[.] [I]t already affected me to just have an account there.
OK to attribute, or no?