The article claims it includes app visits: "Since then, total monthly visits to Facebook in the US (both on the web and through the mobile app) have fallen [by 3 billion/month]."
SimilarWeb has no fool-proof way of tracking app usage, nor even app downloads (don't trust anyone that says they can, unless they're Apple or Google).
What they can track is the number of ratings - nothing else, but there's very little correlation between that and app usage.
