I would imagine that on a large enough scale, analysis of photo contents, timestamp, location, and correlation with any other FB/app activity at that given time

I would imagine that on a large enough scale, analysis of photo contents, timestamp, location, and correlation with any other FB/app activity at that given time/location can provide a ton of new data points to add to their behavior modeling algorithms. There may not be obvious one-to-one connections that we would make from the information we see in a photo, but the more data points that can be fed into a model, the more accurate the model becomes.