To start with, you can reach us on diaspora* at firstname.lastname@example.org and on Pleroma (i.e. Mastodon-compatible) at email@example.com. (Those are not email addresses, although they certainly look like it. They're OKSocial HQ's diaspora* and Pleroma accounts; you can send a message to them after you sign up for an account of your own ...)
You can also reach your local administrator on any OKSocial diaspora* server as podmin and on any OKSocial Pleroma server as admin.
If you're having trouble registering or logging in to an OKSocial server, please email us directly.
If our system is completely offline, we'll probably be very busy trying to bring it back. In that situation, you can reach us or get updates on Twitter @OKSocial_HQ. You might want to go ahead and follow that account, since in a catastrophic incident (like a PG&E planned outage, for example) this server will probably also be down ...
We do also have a facebook page, which we use mostly to run ads on facebook telling people that there are alternatives to facebook. It's not really a good way to contact us; frankly, it's more like performance art.