In the case of the SH instance the government seems to be the only user, while on social.bund.de you’ll find a flurry of different stuff, both state and federal level, say the federal weather service, unsurprisingly quite popular. I’m actually quite surprised Bavaria of all states is using the federal instance.
Instance choice is going to be complicated with some SH institutions because on the one hand we’re just as much as a “state’s rights and sovereignty” state as Bavaria (pretty much the only thing we have in common short of agriculture), but OTOH share quite a few with Hamburg, say, statistics.
These things are never as easy as “just open an account” when it comes to states.