By choosing images over links, and by restricting markup, Facebook, Twitter and Google+ are hostile to HTML. This is leading to the plague of infographics crowding out text, and of video used to convey minimal information.
The rise of so-called infographics has been out of control this year, though the term was unknown a couple of years ago. I attribute this to the favourable presentation that image links get within Facebook, followed by Twitter and Google plus, and of course though other referral sites like Reddit. By showing a preview of the image, the item is given extra weight over a textual link; indeed even for a url link, Facebook and G+ will show an image preview by default.
Consequently, the dominant form of expression has become the image. This was already happening with LOLcats and other meme generators like Rage Comics, where a trite observation can be dressed up with an image or series of images.
Before this, in the blogging age, there was a weight given to prose pieces, and Facebook and Google preserve some of this, but the expressiveness of HTML through linking, quoting, using images inline, changing font weight and so on, is filtered out by the crude editing tools they make available.
Feeds and feed readers started out this way too, but rapidly gained the ability to include HTML markup. Twitter went back to the beginning, and added the extra constrain of 140 characters because of it's initial SMS focus. Now it is painfully reinventing markup, though the gigantic envelope and wrapper of metadata that accompanies every tweet. This now has an edit list for entities pointing into it, and instructions for how to parse this to regain the author's intent is part of the overhead of working with their API.
Image links, however — at least those from recognised partners — are given privileged treatment. Facebook and Google have emulated this too, leading to the 'trite quote as image' trope. The spillover of this to news organisations became complete this year, with blogs and newspapers falling over themselves to link to often-tendentious information presented in all-caps and crude histogram form.
So here's my plea for 2012: Twitter, Facebook, Google+: please provide equal space for HTML. And for authors and designers everywhere, stop making giant bitmaps when well-written text with charts that are worth the bytes spent on them could convey your message better.