It doesn’t matter whether or not you’ve heard of Reddit or ever visited it. If you watch the news, check your Facebook feed, or see a meme on a regular basis, the chance that you’ve seen content that originated from or spread as a result of Reddit is nearly 100%.
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: