How this things work ?
When you send a message to "73s" using your APRS setup, the 73s computer will pickup the message that you sent and relay it out to Twitter. If you want the message to appear on Facebook, you need to authorize twitter fo facebook app. Make sure your twitter account authorized for 73s.org first. Simple is it ? take your time to figure it out. For more information, visit http://73s.org.
