Do you wave/nod/say hi to other cyclists?

Most of the time; tends to be done between 'roadies' though.  Was on my MTB once and I got nothing (from the 'roadies')! 

I say hello to pretty much everyone including times when I'm full out sprinting or "on the rivet" for a length of time.

Barely 50% acknowledge it but why should I care? It's nice when they do, else maybe they saw me too late/are in a world of their own/are suffering etc etc or just maybe they are miserable.

Don't take the knockbacks personally, but feel happy when they return a greeting if you want!

I either raise a hand from the handlebar in a sort of half wave or give a head nod. I don't bother doing it in the city centre if it's busy, but if it's quiet then I acknowledge.

I save 'hi' for when passing a cyclist going the same direction as me. It's very important that the 'hi' sounds really relaxed and not as though you're out of breath.

Lately I've also been smiling at pedestrians waiting or crossing the road - that way they know that you've seen them and that you're not planning on running them over.

Dr Hutch has given us a handy guide

https://www.cyclingweekly.com/videos/watch/dr-hutchs-guide-waving

I tent to follow the nod and finger raise (I'm quite old)

I acknowledge all other cyclists unless I am in a difficult position such as negotiating traffic in a built up area. As someone used to say on another site I used to use, if you see someone without a smile give them one of yours. 

Depends.

When out cycling on road I will give a nod or wave to any cyclists I see. If I see anyone who isn't a "cyclist", but a person on a bike, or indeed walking, it depends on what reaction I think I will get, from saying hello to familys out on a ride, through to saying nothing to groups of kids or adults who are out pottering around, although I will acknolage them if they do me. I used to but it is met with confusion or being ignored. 

When out off road on my mountain bike its often a hello, if not stopping for a quick chat about the trail, any interesting routes, where they are headding etc.

When riding within Oxford I obviously don't, as it would be every 10 seconds and a bit odd.

I nod to just about everyone when I'm out for a leisure/training ride. No nodding on my commutes (not that I've been commuting recently...) though.

I do generally, and to others - drivers, pedestrians and motorcyclists if i'm near or interacting with them. If they acknowledge, fine - and plenty do - but it's my choice to nod or raise a hand, and not their responsibility to respond. Wouldn't sweat it, plenty of reasons why something doesn't come back, not worth judging or second guessing.