I would look at numbers here in slightly different way. 400 people are part of a group. Facebook allows anyone in group to add anyone else into the group even without the person's acceptance. So this is kind of sending 400 invitations out for an outreach. On most of the messages many people in the group were prodded for attention (@mentioned) and on average "8%"  have replied and participated (made initial edits). This is akin to conducting the actual outreach. Rentention is something that needs to be seen after a few months if these users continue to edit voluntarily (having attended an outreach event).

In all these, I probably don't like the prodding part (in real world I would compare it to shouting "Wikipedia Wikipedia" (like tea,coffee) outside the Wikipedia stall), *after most people know* that there is a stall (group). But am not sure how else it would be possible to attract attention of people on facebook (which is meant for connecting with friends primarily).

On the challanges part, I would like to know, how it can be scaled beyond a person's network. Since people don't join the group by self, amount of people who can be reached is low and adding random people from friend's list into group isn't going to help.

Theo, To be fair to OP(Original Poster) , I see the mail only as an update and didn't seem like any boasting was done.