iPhone query….

[CurlewKate]

I’m probably asking a ridiculous question that everyone but me knows the answer to, but if you’re on,say, Mumsnet, could a Facebook friend see that you’re active online?

only if you are on Facebook too

Do I have to be active on FB or just have it open?

You can turn off that function on fb if you want. If you have fb open it will show that you are ‘active’.