The movement of a majority to gmail makes some sense. Android (and all other google services) basically forces users into making a google/gmail account. Not to mention people tend to flock to the same services once they start snowballing in popularity.

Before gmail, everyone (that I knew) was on AOL, which (probably) got its users from requiring accounts to use their network back in the day. I don’t remember it that well though, so I might be wrong there…