When is a social network not a social network? When "using" a social network is a requirement to access services you really wanted to use. Like email. Or videos.
This is common sense to most people, but not at Google, which spent years insisting that its Google+ ghost town was really a thriving metropolis, in large part because people who used other Google services were using Google+ to sign onto those services. Then again, for years, Google users haven't had a choice — Google insisted that they use Google+ when they used other services.