Sites like Facebook force us to skim rather than engage with content: we only glance at a photo before "liking" it, and similarly we don't actually read articles before hitting the "comment" button. The model of these social media sites therefore leads to shorter attention spans and a decline in critical thinking.
Facebook, twitter, snapchat and instagram are all major social media platforms that thrive on short-form information sharing. Twitter has characters limits and snapchat automatically deletes photos after a period of time. People who use these apps often become used to getting all the information they need in a sentence or two or even just a photo. These are platforms for communication, which often involves one's opinion so comments are abundant. The problem occurs when a long-form news story makes it into their feed and they feel they can gain enough info just from the title before saying something that may be completely off base.
Attention spans are so low now, and I believe they have gotten worse because of social media. This is because social media is often instantaneous with responses from friends, family, and others. People don't feel they have to pay any more attention than to the headline because they have limited interest.
It is not the fault of Facebook that the human population likes to read as little amount of text before forming an opinion. This is why, I think, authors put a small article on the back of their books: so people can get a quick idea of the subject matter. This is not a new concept unique to Facebook's content.