- Esperanto: 1.3 thousand. There are also several other smaller groups organized by country or subject.
- Ido: 61 members.
- Volapük: 34 members. (This one is actually called "Esperanto - Volapük", so one deduce it's not purely in volapük).
- Interlingua: 10 members.
- Interlingue (Occidental): Group not found.
- Novial: Group not found.
- Esperanto: 239 thousand.
- Volapük: 120 thousand.
- Ido: 27 thousand.
- Interlingua: 20.5 thousand.
- Interlingue (Occidental): 3.6 thousand.
- Novial: 1.6 thousand.
Volapük — in second place — is surprising. I've read somewhere it's actually a robot that has translated many of the articles, but I don't know whether it proceeds.
The Wikipedia version denominated Simple English has 124.7 thousand articles, but as it seems, it doesn't use exclusively Basic English, for that reason it wasn't listed.
There's also a Wikipedia version for Lojban with 1.2 thousand articles (also surprising), but Lojban seems to be much more a proof-of-concept than an auxiliary international language.
The conlang called Elefen has 34 members in Telegram, but once it hasn't a Wikipedia version, it was not included.
Esperanto, for sure, is the most spoken conlang by far. One claims that there are more than 500 thousand speakers world wide. Ido is supposed to be in second place in number of speakers, and it is said to have a few hundred speakers.