Some people, including in these comments, make the argument that preventing E2EE on Facebook is pointless because offenders can simply move to another service. That's not a very good argument for two reasons.

First, Facebook is already used by pedophiles despite the lack of E2EE. 2500 arrests for child abuse last year according to the letter. An example is given of a pedophile identified through his messages with an 11 year old girl. With E2EE enabled he would not have been identified at that time.

Secondly, the letter highlights that a major problem is the combination of open profiles of children with E2EE messaging. This does not apply to any service, mainly Facebook and similar platforms.

Don't get me wrong, I am overall in favour of E2EE, and I share the usual cynicism about the government's intentions.

You can make the argument that although absence of E2EE does help identify pedophiles more easily, in the balance it is more important to enable E2EE to protect privacy.

But you should not make the argument that absence of E2EE on FB would not help identify pedophiles more easily, because this letter provides evidence that it actually does.