the user ID in the forum is an easy to use tracker of how many people joined the community
Very true, but you only 'register' on the community once you actually go into the forums etc. Which means you must have logged in and then spend time on our boards.
Like I said, unfortunately not everyone does this.
Which means these people have signed up for a sim, activated and are happily using it, but they never show up as a forum ID.
A good example of this is my wife, both my sets of neighbours and my parents in law, who I've all signed up, but who will likely never show up on the forums.
So that's 7 people right there that you'll never see and there are a lot more people like this.
It's not the way giffgaff would like to see it working as we'd like to get everyone involved, but of course not everyone is necessarily interested in getting so involved with their mobile company.