Make a website where each user has an RSS feed and posts/reposts stuff to that, mainly links to things they've found interesting. "Following" someone means subscribing to their RSS feed. That'd be federated out of the box.
I showed up to the internet party around 2011, a little late for my age, so I was still getting my legs and kind of missed "being there" for a few events, like Megaupload going down and Google Reader getting killed.
I'm honestly starting to get a little stunned at the number of number of times I see people say "yeah, Google Reader could do [this-or-that]".