My mother passed just after I graduated from high school in the late 90s and although it was pre-FB (et al) and she wouldn't have been into social networks, I do wish there was something that recorded who she was...her moving about, her voice, something. A bunch of her friends ransacked her house for her things, which I was told they (my mother and them) had previously agreed upon in the case of her death, and so when I got there, there was almost nothing left of her things. I can't remember her voice anymore so, in that sense, having a FB profile of hers wouldn't do it for me but a Youtube video would. That would be nice.
I'm in a similar boat. My mom passed away in 2001, and I have very few digital reminders of her (mainly just a few emails, but she never really got into the whole internet thing). She wasn't one to be photographed often, and even if she had been, I didn't get my first digital camera until a year or so after her death. I'm trying right now to recall her voice, and I'm having a lot of trouble doing so clearly.
As much as a posthumous online presence could be unsettling, I really wish I had more tangible things to remind me of her.