Here goes nothing.
You're right, we'll have more than Facebook. Facebook is just the first and most popular at the moment. For example, we'll have hubski (things like it), which shares a lot of similarities with my Facebook feed now that I've weeded it. Facebook, I would argue, is extraordinarily representative of our society -- that was my point with the deacon's diary. We're a trivial fucking people, and lying to the future about that isn't helpful from an anthro standpoint. Additionally, I disagree that Facebook is entirely trivial, or that the good will be "drowned out" by the bad. That's a historian's job, making sure the good stuff isn't lost in the haystack.
When you say that Facebook is a trivial place and that "we" share everything that isn't important, statuses about lunch and bathroom breaks and god knows what, you're doing a disservice to maybe 5 percent of the population, and it'll be that 5 percent that matters. I don't want to just use myself as an example apropos of nothing, but I haven't physically typed a Facebook status in at least a year. I share articles on everything from baseball to economic policy (many found on hubski), and the comments on them could theoretically provide a pretty solid snapshot of, say, how relevant certain issues were to certain people (from a future perspective). Facebook isn't one entity.
We got really, really, fucking lucky with Shakespeare and you know it. I consider him the greatest user of the English language to ever live, and it makes me wonder if there were others just as good who we don't even know existed. (Some say, for example, that Marlowe was Shakespeare's equal, or would have been. Who knows now.)
So on balance, it's a null. Although why we "friend" people we don't respect is a bit of a mystery, I'll admit... while also pointing out that it's a big part of the problem.
And without Facebook, history would forget the bullshitters. The 500-year-post folks?
I guarantee that they have outlets other than Facebook.
It's a null for me, but who cares. I certainly don't friend anyone. Occasionally people request me and I accept them if I don't want them to get annoyed with me in real life, where things matter.
My overall point is that from an anthropological standpoint of learning about our species, we don't want to "forget the bullshitters." We want to know everything. I've studied Roman daily life -- what they ate, why, when, how they went to the bathroom, how they treated their slaves, wives and kids -- and do you think that's the sort of thing they found interesting about themselves? The point isn't that without having Facebook from 1500 on we'd have missed out on Newton's Principia -- the point is if we'd had it we'd know a lot more about why he was so fucking nuts all his life.