Product

The primary reason most people use social products (and stick around) is their friends are using it. This is why invites and contact upload are core UX to any successful social network.

The secondary reason is there is entertaining or interesting content. This is why algorithmic feeds have won.

Nothing else matters.

I do believe there is a 3rd that you are way over discounting:

3. Making Money

True for Twitter, LinkedIn, Instagram, Warpcast

and made even more powerful with onchain social

+1 to making money (ie, Behance for creatives)