Right. Assuming that they know that they're missing 1% of cases insinuates that they know enough about the 1% they're missing to quantify it.

Isn't that right based on the statement though? The implication is 99% of the identified cases on Facebook are caught by Facebook's systems, the 1% not caught by Facebook's systems are tracked by other means. 100% of identified cases, not 100% of all cases.

Still very misleading if that is the case, what other means are there realistically to catch something if not by FBs systems, on FB? Self-reporting in the groups? Is that counted towards the FBs 99% or the 1%?

Reported directly to the police or other authorities? It's probably quite realistic in those scenarios for something that Facebook's systems have missed for it to be found by someone eventually and escalated.

There's a little ambiguity in there, but I wouldn't go so far as to say it's "misleading".