People just message jumpers they know at DZ's or message the DZ itself on Facebook. Pretty quick and easy answers since most DZ pages have multiple admins, so someone is sure to check their phone and respond even outside of business hours.
I noticed that a lot of local DZs post "this week's hours" on their private facebook pages, and that is how I know where to head after work in the summer time. In the winter here in the Midwest most DZs shut down, yet we are jumping a Cessna every weekend, weather permitting, but all the Chicago jumpers don't know that so we end up shutting down early because people only do two or three jumps before they freeze, so we would need more jumpers to keep the plane going all day every weekend. It's not that bad if you bundle properly.