Be careful comparing the "Registered Users" metric on the page between sites. In some cases they're using total number of registered accounts on the forum, and in others they're using metrics that involve recent activity, like a "Monthly Active" metric.

Also, don't trust the numbers, even if they're cited. The current value for Twitter (500 million) is cited as being from an article in 2011 that mentioned 200 million. The Twitter company page - https://about.twitter.com/company - says the current official monthly actives is 313 million.