None of the above. When asked "why", I usually reply that if keeping in touch with someone is of meaning to me, there are many many ways to do it - call me anachronic, but I absolutely love handwritten letters. Also, if thou really are friends with someone, then you can not change a word in a year and it will still feel as if you met yesterday. Meanwhile, social networks tend to be filled with "friends" who thou've last seen half a millenia ago, and even then you didn't really like eachother. Besides, as Traveler wrote, I am uncomfortable with the thought of spreading my personal info.