Earlier quoted context omitted.
Keep in mind that it is possible to infer / derive a lot of meaningful data about you from a relatively small sample size. From political leanings via social networks (personal associations are very strong data points) to samples of your personal writing style (see http://en.wikipedia.org/wiki/Stylometry )
And why is this bad?
(this is in reference to OP stating that they only use social media for posting things they don't mind being public)