Facebook has a number of features for communication that email doesn't have:

1) Passive communication: the fact you can go and check someone's profile and see what they have been up to is very powerful

2) Default public communication. By making communication public by default it lets your other friends and family benefit from any communication you do with a 3rd party

3) No email addresses - people change email addresses a lot more than they should. The only comparable problem on Facebook is when women get married and change their lastname, but I've never seen any problems arising from this.

That really just scratches the surface of the differences.

Sometimes, doing something better really is enough to make a huge difference.