I actively discouraged my 7 year old DD from using the computer when she was little but children seem to be "drawn" to them. Computers are a natural part of their world like books were when we were grwowing up. Our school now has 'interactive whiteboards" in every classroom so even at school everything is electronic. And my DD would rather be on the "Club Penguin" site than watch TV these days.
So I can sort of see that Facebook is the next step but I agree it is far too young. Too much content that is too old for them. But having said that I think Emmy83 is using it in a great way.