Why do people use sites like Facebook and Tumblr as "image macro" sharing sites?

My take on it is this: The Facebook "top news" algorithm favors photo posts and shares over text messages. So their systems have built an incentive to post/share that sort of content. Further, with no ability to control the font or font-size of the posts, these (stupid) image macros catch the eye and trick the user into reading them. The feedback mechanism comes in from the Likes and comments that are generated on this content. These +1's get the content higher in the algorithm, insuring that more people see it and share it. Lastly, the thing you are not realizing is that some 80% of these are actually sharing a post made by someone else. Thus the real number of photos has not sudden swung to text-based image macros; just the number of posts ping-ponging around FB as everyone re-shares something they found funny. In the end, when you can just single-click and some "witty" phrase is shared with your friends, that makes it very likely to continue widely. But I really hope this is just a fad. (Solution! Use FB mobile. The thumbnails of the image macros are too small to read.)

Like pointed out, the actual number of photos has not actually increased - it's just that people are more likely to share them. They're simple to read, often humorous, and don't require too much time to interpret and share (a big plus in the short-attention span world of social media). I too am asking whatever happened to original content. It seems that more and more people are become passive observers - and at best passive sharers - of ideas and messages. I think this is partly due to the increase of the amount of information that we are exposed to. The more information we have in front of us, the more we're overwhelmed to even take the time out to create our OWN information. This is not necessarily bad or good, it's just a widening gap between the producers from consumers.