Search Party or Not . . .?

I have always wondered what to do when people disappear from your life suddenly. Oh, I don't mean they move away or die but just stop checking in with you and sharing trials and tribulations. I also have to wonder about people who check in and out or our lives at will like there hasn't been an hiatus in contact. I know a couple of 'friends' like this. They are constantly on your Facebook/e-mail and suddenly, your last note to them go unanswered . . . for months so you put the friendship aside and set them free! Then, they show up, again, and are constant in contact . . . until they disappear . . . again!