I think it's the other way round - what makes you think these vertical temporary spikes of account creation can possibly be organic growth? Even viral growth doesn't look like that and, again, it's not reflected in the normal activity of the site. Creating accounts in bulk has always been trivial, there was a spam post with 600 upvotes just yesterday.

As to the text posts, I'm not sure I understand its relationship to your original claim, that is an odd jump but it doesn't sound all that difficult to look into, without that we'd both be mostly guessing (my guess is still likely spam but I have no idea). Unless you already have?

Edit: I tweaked the query slightly and I haven't looked into the content but that kinda looks like spam to me? I.e. the number of nondead text stories hasn't changed in years. Spamdetection kills a lot of stuff as you can check for yourself by browsing /newest

https://play.clickhouse.com/play?user=play#U0VMRUNUCiAgICBDT...