Thanks to @trish for initiating a lively discussion, and thanks to @nounishprof for the hat tip 🎩.
TL;DR
casts are kept live in Hubs
casts are stored in databases
casts are exportable as JSONs
threads are savable as Farchives
Farchiver hosts public and semi-private Farchives
Casts are kept live in Hubs
The Farcaster protocol achieves eventually-consistent consensus on live state. Each FID has a storage allocation, and activities that go over the allocation lead to pruning of older activities. More details elsewhere.
Casts are stored in databases
A handful of individuals maintain public databases of cast history. Some will prune, some will not.
The best known example is searchcaster.xyz by @greg. Somebody else (pixel?) received early Purple grants to make their database snapshots public.
Neynar pushes their private custom Replicator dataset to Dune, where it is available to the public via Dune's gating. Airstack provides access to Farcaster data via their own private APIs. Etc.
Casts are exportable as JSONs
It is fast and low-cost (often free) to get your own cast history on your local machine! It is entirely yours, and you can do anything with it, including back it up.
Check out fcau.xyz by @iammathias for an MIT-licensed FOSS Node CLI utility.
Check out fario by @vrypan for a powerful CLI tool in python, also MIT-licensed.
Check out @pixel's guide. Neynar is not free, but the one-month cost is less than one year of Farchiver.
All of these are really fast. The actual process to download full history takes less than ~2 minutes for most users. The setup may be intimidating to some, but is not super difficult for experienced devs.
To find free, open Hub endpoints at which to point these utilities, check out the list we maintain at foss.farchiver.xyz.
Threads are savable as Farchives
Farchiver lets you download every thread you've ever touched, with all interactions, in a delightful package. We suggest Obsidian for browsing; it can be any Markdown-aware knowledge tool.
We believe this is more interesting than just one's own casts. We charge 0.03 ETH for one year of this unique service.
You may purchase a snapshot of any FID.
Farchiver hosts public Farchives
In the public interest, Farchiver hosts Farchives, pinned on IPFS and available for the community to preserve collectively.
Examples:
the proto channels thread, NBA game 5
an epic @keccers.eth thread, requested by @shazow.eth
a month of @dwr.eth interactions, around the wowow meme
Farchiver also enables private, sharable Farchives of threads of interest. Each user has access to a section of private Farchives (possibly encrypted) behind a wallet login. This includes Farchives that others have paid for, to share with others.
As an example, one caster wanted to own the toxic interaction they had with a powerful account. It had been deleted, and only pieces of it had been screen-shotted without the full context. Now this user has a copy of the deleted abusive casts, and this Farchive can be shared with others as a semi-private, but transparent and permissionless record.