I wouldn't do it. I wouldn't worry about the "fairness" thing, it's your private life, you don't need to be "fair."
I do have former students as friends on FB, all but two have graduated high school, those two are seniors and the "friending" was done with parent communication. I won't friend children at all, whether church associates or friend associates.
I have tons of friends who are my current students' parents, mostly because I went to high school with them, so I tend not to post a lot about work, anyway.
FWIW, our district's warning about FB was also about MySpace, e-mail and text messaging: no such communication with current students, ever, for any reason, at all. We have also been told not to reference work in any way on our FB page that could be misinterpreted. So, we can say "I had a bad day at work," but we can't say, "Terrible day: 65% of our students failed the State Test!"