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Hm, that's true. I would guess that most people don't use the web that way, though. And this is all about averages, so as long as other people don't use Ghostery, Facebook will still edit your feed based on their behavior. The only solution is to get everyone to use Ghostery :)

Can the Facebook tab not tell when it has focus?

But it doesn't know if you left the other tab open in the background to read later (if that page doesn't have effective analytics because Ghostery blocked them).

You're right, but with enough data I suspect that becomes a relatively small error source.

It is knowable whether the page was opened that way, by seeing if the browser environment is still around after the click. The analysis could just ignore that set of data.