Yesterday Jesse Stay observed that Twitter and Facebook have both discontinued RSS feeds from various parts of their service. Later that day he commented on the lack of blog reactions. So here you go. Some thoughts, expanded from a comment I left on the original article.
A social service is able to offer a better experience from knowing who their users are and what they are reading. Learning the users interests allows the site to suggest related material, and also target advertising to the specific person. Content publishers in turn can get data about who finds their material interesting, not necessarily identifying the individuals but detailed demographics and related interests.
RSS doesn't fit into that world. The entity fetching the RSS feed is often not associated with an individual user at all, instead being an aggregator or other bit of infrastructure somewhere. Once syndicated via RSS, the originating service loses visibility into who accesses it. The aggregator might report a total number of readers, but not the same rich detail which the service would get natively. Users on RSS are thus far less valuable than users who come to the site, or tools which use the site's (authenticated) APIs. For the originator, tracked activity is much preferred over anonymous content consumption. Paywalls are another symptom of the same underlying phenomena: anonymous content consumption isn't working for the publishers.