One problem of course is notifying the thread starter of new decentralized comments. There were Trackback, Pingback, Webmentions for web pages. For feeds there was the Salmon protocol [2], enabling notification of new comments to swim upstream. According to Google's documentation Salmon would have been an API of Google Buzz. But that started with a privacy scandal and shortly thereafter Google decided they needed their own closed social network. Salmon was later used in OStatus 1, I think.
There was a lot of activity in the 2000s transforming the RSS/Atom ecosystem into something of a decentralized social network. Extensions like Activity Stream for richer data, AtomPub, Salmon, OpenID, Webfinger, PubSubHubbub (now WebSub) to turbocharge feeds with a Publish/Subscribe thingy and a lot more, that I'm now forgetting. The rule of thumb is that the problems stay the same, but the syntax and the name changes.
And yes, if you squint the right way stuff like ActivityPub (Mastodon) and the Indiewebcamp Microformats are practically feeds.