I disagree that it's easy to leave like that. I have the email/phone number of maybe 5% of my Facebook friends. If I need to contact any of them, the quickest way is Facebook.
That depends on how often you do need to contact them. If you don't call/text/IM them, then you could have no contact info and not miss it.
I also think that it is relatively easy to find somebody's phone number once you do wish to call/text them. You can just ask a common friend or Facebook them from a throwaway.
