There’s a catch-22 in social media for those whose job it is to stay active and informed. On one hand, you have to constantly update Twitter, Facebook, Google+, Instagram, and Foursquare; slowing down on any of them can hurt exposure though the various ranking systems (such as Facebook EdgeRank) associated with them. On the other hand, you have to keep them somewhat diversified to both cater to the specific personalities in each as well as give a valid reason to be followed actively on multiple channels. Why would people follow your Google+ updates if they’re just carbon copies of your Facebook posts, for example.
Technically speaking, Flickr IS a social media site.
You share images and videos (yes, you can share videos there, in case you hadn’t heard) and allow people to share and comment on them, so Flickr fits into the minimum criteria for being considered a social media site. With that out of the way, why is the site designed to do this: