I'd wager the majority of them are dead/inactive accounts. I remember graduating from undergrad and thinking I had to make a LinkedIn account to get a job, used it for maybe 45 minutes, and never touched it again. I feel like this is the case with a lot of people
I'm suprised as well. But it says monthly active users. So I'm guessing they count distinct logins per month, which a dead account can't do. I'd guess it's probably bots looking for keywords and building profiles for recruiters.