If Facebook is the minimum for top-tier then there is only one top-tier social network. Being an order of magnitude off Facebook still makes the network one of the most popular social networks of all time.
(Also Telegram and WhatsApp are a borderline case; they have the users, and they have social-network-like features, but most of the users are likely not _using_ the social-network-like features; they just use them as messaging apps).
The second tier is things like Snapchat, Twitter, LinkedIn, Reddit, Pinterest, Quora; these are in the 300-600 million user range, so they're big, but you don't have the same sort of universal exposure.