Imagine FB's data attached (by email or cookie) to Reddit's screentime - would easily quadruple Reddit's ad revenue with the same ad inventory. If they got away with it without driving away Redditors, would be salivation worthy as a marketer.

Reddit's main traffic problem for advertisers is that most users don't really subscribe to subreddits or make accounts, but rather browse the front page casually. So you can target, for instance, /r/smallbusiness to target small business owners, but you'll exhaust that inventory very quickly. A Facebook acquisition would mean that a much higher percentage of Reddit's audience could be served relevant ads.