Twitter has always been "small" but popular with people who work in the media, so you hear it mentioned on the same level as Facebook by those people, even though it's never been any where near the same size
Perhaps "active users" are defined differently? I know a lot of folks with up-to-date LinkedIn profiles, but who rarely interact with the platform --- it's a "set it and forget it" platform for a lot of people. Log in when you want to connect with a new colleague/resume update, but other than that kinda ignore it.