That is a super-project of the type FaceBook or MeWe.

50% of OxWall does not work with more than 10,000 members. I have modified a large part of the core so that it works with a few thousand more, but a million ... I see it too much.

All plugins are going to give problems and you are going to need a super server.

For example, if you have 100,000 members with 1000 friends each, "Friends", "NewsFeed", "Notifications", etc. are no longer useful.

CRON should run from another server to not slow down everything or even crash, because if it runs every minute and takes more than a minute it will step on itself, so you need another type of solution.

Orphaned records (typical of OxWall) are going to be millions.

The user photo folder saves those of all members, if you have 500,000 members with 1 or 2 photos each one already breaks out there, you would need a folder for each user, that is, the Photos plugin does not work for you either.

Neither MailBox, which retains all conversations indefinitely.

You will need several good programmers working 8 hours a day for you. For such a large project you need a good initial budget, hire a programming team and trust them to recommend that you use or develop a new one.

OxWall's internal architecture is based on making queries to the database, loading it into an array and working with the data obtained, but any query can leave you without memory.

I think OxWall can help you get started and while developing another kernel more optimized for large volumes, with another folder tree, with everything new.