When Google+ was shutting down, I did estimating of the total amount of public content within the Communities feature. Median size of a post was pretty close to Tweet-sized --- 120 characters or so. (G+ could ingest very large posts --- I never hit a limit though I wouldn't be surprised if it was book sized.) The highly active user population was maybe 12 million (another 300m or so posted at least once). Volume seems to have been about a million posts per week, for six years.
Which works out to less than 100GB of actual user-contributed text.
https://old.reddit.com/r/plexodus/comments/afbnvd/so_how_big...
Adding in the rest of G+ multiplied that out a few times, but likely still well under 1TB data. Images were asssociated with about 1/3 of posts and weighed in at a few MB each, call it 3 MB --- so a picture is worth 24,000 posts.
Rendered and delivered, page weight was just under 1 MB (excluding graphics), for a payload-to-page ratio of 0.015%. (Ironically, about the same as the ratio of monthly posting users to all accounts.)
But if you wanted to extract and store just user content and metadata, excluding video, storage requirements are surprisingly modest.
Facebook data aren't clear, but:
> There are 2.375bn billion monthly active users (as of Q3 2018).
> In a month, the average user likes 10 posts, makes 4 comments, and clicks on 8 ads.
> Hive is Facebook’s data warehouse, with 300 petabytes of data.
> Facebook generates 4 new petabytes of data per day.
https://www.brandwatch.com/blog/facebook-statistics/
I'm going to assume much of the 4 PB is system data, not user-generated.
2.4 billion users posting 4 x 120 byte comments/mo works out to ~15 TB of textual data per year.
Values are extrapolated, unconfirmed. Corrections or suggestions welcomed.