Any book released any time anywhere might get thrown out. That's the risk we take releasing in the wild. I would think that a well-labeled book that clearly says Take Me or I'm Free or something of the sort would be less likely to get tossed, but who knows really why anyone would ever throw out a book. Personally I can't imagine doing it, but people do, all the time.
As to friends and relatives, it's just impossible to instill a love of BookCrossing into anyone who isn't intrigued and delighted in it on their own. My brother is a great reader, a published author, a college professor of both writing and literature. Couldn't think of a better candidate to appreciate BookCrossing. But he doesn't. He has no interest in it, as much as he loves books and reading. And if I give him a registered book, it remains unjournaled. I've just had to learn to accept it.
The same with friends, some of whom have actually signed up with BookCrossing, even one who has released books herself. Can't get a JE out of them when I give them books that they are interested in, even ones they ask for.
BookCrossing teaches patience and acceptance. Or you get ulcers. : )