Yeah, the RSS ecosystem was never the leading ‘social network’ by number of users, but it used to be systemically important, in a way that it isn’t today. Keeping track of what was happening in many fields (particularly tech, but also others) used to be primarily via RSS, now Twitter (and in some cases moving to Mastodon, now).
Google Reader was shut down in 2013.
At that time Facebook was THE hugely dominant social network, with Twitter solidly established, and Instagram rapidly becoming popular.
Google Reader was well past it's prime, most "regular" users had already moved to those platform, outside of the more hardcore Tech circles.
Google Plus was also really struggling,so they probably wanted to push the user base on to their "real" social network.