Well, I usually lift a few fingers and nod.

Most respond, some are in their own world.

Sometimes, I'm in my own world too. Paying attention to what is in my path, other thoughts in my head - may not even notice someone passing in the other direction.

I almost always ask a stopped cyclist if everything is OK.

Interestingly, when I'm riding with my wife - who rides slowly - sometimes, I will stop - standing over the bike, waiting for her to catch up. If I wave at an oncoming cyclist, most of the will start slowing and ask if I am OK.

I only ride on MUP, the section I ride is 90% cyclists.