It's long been held that one of the best ways to reach your followers and create engagement was with visual content like photos, but now the best visual content is a video:

Looking at more than 670,000 posts by 4,445 brand pages (not including celebrity, entertainment or media pages) between October 2014 and February 2015, Socialbakers found that video posts had organic reach of 8.71%, meaning an average of nearly nine fans out of 100 see such posts. The reach was only 3.73% for photo posts, well below the results for text-only statuses (5.77%) and link posts (5.29%).

The video number reach is less surprising than the fact that text only and link posts surpassed the reach of photo posts. It will be interesting to see if that trend continues. 

But for Facebook, this seems to have been part of the plan:

As recently as last April, Socialbakers reported that 75% of all content brands posted by 30,000 brands included a photo. Photo posts were driving major engagement — 87% of interactions on the top 10% of the brand posts Socialbakers studied then came of photo posts.

That led many Page administrators to turn to photos as a kind of link bait, using photo posts to drive reach and including links in captions hoping to get clicks back to sites. But in August, Facebook announced that it would crack down on that practice, favoring posts using Facebook’s native link format over the photos-with-links-in-captions workaround.

So it's not to say that photo posts aren't effective, but it's not as simple as posting a photo an ensuring your content reaches the desired audience. Here is a visual of the numbers for the 4th quarter: