You know there's stuff employers cannot ask you legally, right? Things like:
- How old are you?
- What is your religion?
- How many children do you have?
- What is your political affiliation?
The point is that these are all legally verboten questions because your age, religion, children or politics have no bearing on your ability to perform a job. (Obviously, if you want to be a fundraiser for Democrats, but you are a Tea-Party Republican, some of these categories might be relevant--but not very often.)
However, you can be discriminated against because of your age, religion, politics and so on--if a prospective employer notices your Facebook profile, your tweets, or other social media and dislikes what he or she finds there. At least, there have not been any statutes or case-law challenges to the practice. And anyway, who is to say whether an employer passed you over "because" of a statement on your Facebook page or not? How can they prove it?
Moral of the story: lock down your privacy settings, and be sure that anything "the general public" has access to presents you with only information you would be comfortable disclosing to a prospective employer.
(PS: Thanks and a tip o' the hat to Joyce!)