While that’s a common feature I wouldn’t say it’s a defining one. There are both historic examples that are squarely considering “social media” without a feed at all (MySpace, early Facebook, etc) and modern examples like Mastodon where there’s no algorithm behind the feed.
As for YouTube, it was pretty clearly always intended to be social media. There’s a reason why there’s no real delineation between creator and user accounts. Even if you’ve never looked at your own YouTube profile page it’s still there waiting for you to fill out your bio and upload a quick video for your actual friends to watch. It was always supposed to be participatory, not viewers just hoovering up content from “creators” and fossil elements of the site design still reflects that.
