Part of this is their capability of processing image posts and giving it a text description. For example Facebook will look at a jpeg of an old man with some te

Part of this is their capability of processing image posts and giving it a text description. For example Facebook will look at a jpeg of an old man with some text on it and use image recognition to actually determine a description of "photo of old man saying 'top text bottom text'". I've seen these descriptions appear in the page and they're pretty impressive accuracy-wise. Beyond that, comparing posts for similarity is likely mundane stuff like who wrote it, associated words, the type of people that engage, and how.