!umehack primarily uses #IRC for communication. External communication (non-members) is via e-mail. We don't however have a mailing list; we used to, but it was never used.
If you get people to subscribe their RSS feed readers to the hackerspace's group feed, or generally become active on !gnusocial, I think it's a great way to "update on what's going on". But not for calling to meetings or anything (then email is best). :)