Eugene hit the nail on the head. Oftentimes, texts & data will get through, whereas a call will not. And, calling 911 from a cell phone, they may have to reroute your call, several times. It can literally save your life, if needed (texting/posting, not the call).
That being said, its the responsibility of the person sending it, to communicate their problem correctly. You cannot gauge the response you will get-you may get an overwhelming response, or none at all. And, as for someone stalking you, or intending you harm; IMHO, I think that ANY friend request you accept, would be somehow vetted. I know about 90% of the people on my facebook account personally-the remaining ones are people I share common interests with (mainly bagpipers). I know that people friend-farm their accounts-I never saw a need for that. I DO use it as a communication tool quite often, and could certainly see using it in dire situations. Ultimately though, the poster is responsible for any & all consequences, good OR bad, as the result of it.
Remember-if they are sending search teams out, these people are being pulled from OTHER jobs, and often do that voluntarily. They are putting themselves at risk, looking for you. Being an ex-member of the SAR community, I think a lot of people who get lost, seem to forget this. I was a big proponent of fining people for carelessness-which happens all too often.