Missionaries need to network.
It was ironic when a missionary friend of mine asked me to describe what Facebook was. His son and daughter are both on there as my friends even though it has been decades since we have seen eachother in person
Missionaries depend on networks of people praying and supporting them. There are so many ways to creatively do this with the Internet. Facebook just happens to be one of the best. Missionaries are finding it increasingly necessary to connect with people directly since MOST CHURCHES ARE NO LONGER INTERESTED in supporting RANDOM missionaries indefinitely. That's the subject of another post.
Anyhow back to my story, soon after I told my missionary friend of Facebook, I got an alert.
A mutual friend of ours had read my one line "status" update on Facebook that indicated I was with this missionary for the morning. She promptly wrote on my wall that she wanted his phone number! I got that email on my phone and put them in touch! They hung out before he left town.
You can't get a better, more personalized explanation of why missionaries should be on Facebook.
Now, all you Twitter people will say that it is better. I agree (sort of), but twitter is much harder to explain and seems to take a lot more action on the part of the user to build a network.