Many years ago, in chatrooms, double-colons were used to set off actions, as opposed to things you were saying. Actions were written in third-person
::opens his backpack and pulls out a small lantern::
The first place I ever encountered double-colons was in AOL's Red Dragon Inn, back in 1995 or so, when I first got a modem. I still use them in chatrooms today, well, except ones that recognize /me. Maybe I'm just a sucker for mid-to-late 90's alternative culture.
Easily much classier than emoticons.
One more example of forgotten netiquette.