Young adult dating sites
Unfortunately in my experience Ok Cupid has become a bit of a ghost town.
CMB was constantly "gently" reminding me to message people I'd matched with.
Changes in the last few years have made Ok Cupid a bit more like Tinder (both owned by the same company) focusing more on swiping and eliminating the ability to message a user without matching with them first.
You can still send a message -- it just won't show up in the recipient's inbox unless you match.
It's a cool concept and helpful for people who want to meet someone in a more organic manner.
That said, I've never met a single person who actually uses the app.