In todays ‘everything connected’ environment you need to be careful about what you say online. It can have knock on effects in terms of what people think about you, especially in the more serious aspects of life such as your career. As such, personal branding is the key, and you should consider everything you say online a press release.
In terms of my social presence I have Stacked Notion (this site), which I use to post technical and social ramblings. @lucaspiller (my Twitter account), which originally had the same intentions however has slightly divulged. Lastly Facebook, which I just use for personal stuff. I have a few other odd accounts here and there, however those are the main ones.
Facebook is my biggest concern, as it is rather well known for its lax privacy model. At least Twitter is upfront that you have absolutely no privacy (it is kind of like The Sun vs. The Daily Mail debate), where as Facebook tries to pretend you do. At the moment I think I am doing fairly well with it, I have my security settings set to show practically nothing unless you are my friend or a friend-of-a-friend – I don’t even appear in the public search results.