I love getting bookmarks for presents but I always warn people to give me disposable bookmarks as they tend to get lost. I also use anything to hand as a bookmark because I don't like to dogear my books. So I use train tickets, receipts and advertising flyers as bookmarks, until they disappear.
I'm not sure what happens to them but perhaps they they get left in the books I was reading. I've just found a wonderful article Found in Books from Abebooks, detailing some of the more unusual objects they have found being used as bookmarks. These included dollar bills, a hand-painted handkerchief and a list of scheduled operations at a hospital. The article is well worth a look.
I suppose the moral is: don't use anything as a bookmark that you wouldn't want other people to see.