I've noticed since I've been on facebook that the number of emails I get per day has gone down. Facebook is very useful for sharing thoughts and information with friends and family, when the importance of someone seeing your message isn't a priority. If it's a friend or family member and the message is important I'll either use FB mail or regular email, though I feel regular email is more reliable and its more likely that someone will check their email before their FB page.
Though I find myself using regular email less and less, its still the best way to send/receive attachments as well as to get periodic notifications, newsletters, ads, etc.
For immediate communication and for short question/answer engagements, so typical in a work environment, IM works the best. But there's no gaurantee I'll be right there to get the message. The same goes for SMS messaging.
But if I really want to communicate with someone right away where I want confirmation of my message and the priority of the message is high... and call me old fashioned if you wish... I use the phone. The best thing about phone calls is that you get the message across, and you know the recipient receives the message if you are there talking to them face to face. Even though you might not get the response you need right away, you usually come away with getting a commitment to the response right away.