Right now, it's not heavily used. Either it's a feature the site has no use for in itself, or it's a feature that doesn't meet the site needs.
Well, I'd kick off with recurring events and joinable events as a start.
I'd also look at the fact that there is a whole swath of data that could be tied into the calendar and isn't: posts. Surely it's a natural interface for 'today's posts', 'this month's topics' and similar...?