Re: Policy via social media: It's a thing now
With respect, I think that you are drastically overestimating how much has survived from previous centuries.
You appear to be assuming that something like a majority of all paper communications from every source survives multiple centuries. This is very, very far from being the case. The substantial majority of all paper documents are destroyed within decades of creation, a miniscule percentage survives centuries and this is why diaries like the Samuel Pepys diaries have been so valuable, illustrating what a single person thought about contemporary events.; Fragmentary sets of letters become deeply important primary resources and are published by the tens of thousands.
Even if you assume that nothing but comments pages on El Reg, The Guardian, & The Daily Mail survive two centuries (and these are already being archived by just the UK national archives) then this contain thousands of times more data about what normal citizens thought about current events survive that was the case from two centuries ago and this would be enough in itself to prevent there being a "historical dark age".
If something like Facebook or Twitter gets archived? Obviously there are tons of dross, but it'll be historically invaluable for future historians wondering what the common people thought at the time. The loss of some data on random digital media is probably in fact not going to be a major loss, simply because statistically most of this sort of stuff is lost anyway.
And this ignores that a few archives will no doubt survive; they always do. Even if it's people passing down dads/grandads/etc collection of family history bits and some random files that nobody ever looked about because they didn't really give a rats ass but realise that it should be passed on, and besides it's only a few gig on a terrabyte drive now, and then an exabyte storage crystal (or whatever) in a centuries time, this being the then equivalent of small USB stick) and nobody can be bothered to sort out that random stuff that takes up practically no space.
Up until somebody does look in a few centuries and it turns out to contain an IMAP download copy of great, great great great great grandfathers email account back when, which contains plain text copies of most data inside of the messages as well as awful HTML formatting.
Hey presto, a huge quantity of data survives to be a goldmine for a historian in the far future.