I'll be honest, this was a long time ago, but I totally used some dating sites maybe eight or nine years ago, but only for a couple months and I never went on any dates. I get the feeling that you're asking about these mobile apps everyone has now. These were "just" websites and not apps back then, but I did visit them in bouts of public loneliness staring at my phone (which were often at that time in my life). Sorry if that makes this a pointless contribution lol.
1.) When you message someone for the first time, do you just send something simple like "hey" or "hi"? Or do you edit it specifically for each person?
I personally always tried to write at least a paragraph that had something to do with the things mentioned in their profile. Sometimes I would go all in and try to write something clever or meaningful but I think it was probably always too long, looking back. Very long.
2.) When someone else messages you for the first time, is it usually just a simple "hey"?
I really didn't get very much back from what I put out there. On "OK Cupid" and "Zoosk" I don't remember receiving any messages period, whether in response or from someone at random. There was a 'geek specific' site called "Geek2Geek" that I did get a couple messages on. A specific one I do recall was of some substantial length, about 3 paragraphs, mostly about themselves, and what they look for in a significant other. I did not respond to this message.
3.) If someone else messages you first with "hey", how do you feel about it? Do you wish they would use something more interesting than that to start conversation? Or is it fine?
Primarily I would feel anxiety, since it really puts the onus on you to reply with something other than a one word reply. Typically in a real-life situation when meeting a stranger everyone generally at least attempts to offer topics of conversation through introduction.
4.) Do you think you would be more likely to respond to a simple hello, or something more complex? Is it weird if someone sends something more complex than "hey" as the first message?
I would definitely be more likely to respond to a lengthier message--I think that it's more strange to expect meaningful return on such a small effort
5.) How many people have you held a conversation with over these apps (estimated)? How many would you say were successful, and any indication to why?
No one. Looking back, they were probably unsuccessful because it was obvious that I was trying too hard. There's a pretty wide gulf between a casual conversation and an unsolicited letter that I refused to acknowledge in my younger years. I also acknowledge that there was probably just not as many people using it as you would find today on something easier to use.