1) I would generally use BBSes daily. Probably for an hour in the evening or however long I could tie up the family phone line. I would log into two or three, read and respond to messages. Very much like reading email at the end of the day. I forget what I started with, but once I found ZTerm for Mac I did not go back. https://en.wikipedia.org/wiki/ZTerm

2) I'm honestly not sure how I found my first. Possible I found some number at the local library, possible a friend in high school gave me one. Once you found one, you could learn about others from that one. There was a real culture of different sysops trying to get you to use their board (but only up to a point because it used phone lines)

3) It's really difficult to know. There was no equivalent to web search or a way to see what everyone else was doing. I'd expect the popularity followed some kind of power law, but everyone used a different subset of boards.

4) It's difficult for me to remember. Probably much shorter messages that would seem "simple" by today's standards.