Accounts are very different from "users per month". Having an account means someone has actually created a login with a password and has information stored. Depending on the website/app, many users will simply not be logged in at all.
Imgflip now has 3-4 million accounts, but public tools like SimilarWeb here (https://www.similarweb.com/website/imgflip.com/#overview) estimate around 12 million visits per month. Many people don't come every month though, so if you stretch it out to 1 year then Imgflip has 50-100 million users.
Imgflip has been used by many celebrities, presidents of the United States, and probably hundreds of millions around the world at this point. In terms of reach, I would wager that half of the humans on planet earth have seen an Imgflip meme. People create on Imgflip and then share virally across all the other platforms.