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Exactly. Facebook probably sends 302 Temporary Redirect, counts it as a click, and the bot simply does not follow the Location header.

Are there any ways to avoid this?

They should put a web bug on the target page to confirm the redirect was completed.

Except that the contract prohibits doing this sort of "third party audit".

I mean Facebook should put a web bug on the target page.

But Facebook is benefiting from the problem! At least in the short term, until it harms their reputation in the advertising market.