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.
The top tier is, essentially, Facebook, Instagram, Youtube and TikTok. These are used by literally billions of people; with the exception of China, virtually everyone on earth is exposed to them fairly directly.
(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.
It's a stretch to call YouTube a social network along with the messaging apps. Facebook & Instagram are two products by the same company where social identity is literally shared across them. If your top tier is Meta + Chinese apps then we're just going to have to disagree.
"Popular" isn't quite the right word—"significant" might be a bit closer? In my country, at least, Twitter was adopted by the political, celebrity, and media class far more than Facebook ever was.
