I don't think the user agents under discussion are the UAs sent by the Facebook browser, but rather the ones sent by Facebook's crawlers. The Facebook browser

I don't think the user agents under discussion are the UAs sent by the Facebook browser, but rather the ones sent by Facebook's crawlers. The Facebook browser is distributed across all of humanity, so it'd be expected to have legitimate activity during the outage. The Facebook crawlers, however, are presumably all behind the blackholed Facebook network, so any request purporting to be from a Facebook crawler is likely up to no good.