If you are concerned your friend is imminently planning suicide, you can call the police (look up the non emergency phone number) and ask them to do a "wellness check". My wife had a coworker who hadn't been heard from for a day so two women from the office had a wellness check done with the police and they found the coworker incapacitated in his home. See: http://www.ehow.com/facts_5823955_police-welfare-check_.html

also, depending on the circumstances and your local state laws, you can have your friend involuntarily committed for observation to a psychiatric facility, but a judge will likely have to be involved. To me, the Facebook post would be enough to seriously consider both of these options. (my wife is a social worker)

these may seem a bit extreme, but they are there for your friends protection. Its better to possibly lose your friendship and him live then having him end his life.

I hope this helps the immediate time being