> At least historians of our era will have terabytes of rumors in Facebook/Twitter to examine. (Or will they?)

I'm almost certain they won't. IIRC, Facebook and Twitter are very resistant to scraping. Eventually they'll shut down or pivot, and all their existing data will go poof.

Plus people think about social media differently than newspapers. Newspapers were an open public record, and some effort was always made to archive them (e.g. the local library binding them into books or microfilming them). Social media is this weird amalgam of public and private, that people are more jealously guarding from the public.