It's a well known fact that individual socks seem to
disappear somewhere in the cycle of washing and drying,
resulting in at least one sock which cannot be formed into
a useful pair. Where these socks go is of considerable
debate, but a few theories have been put forward:
The first is that the socks are simply left and forgotten
somewhere in the washing process. For instance, a sock is
left on the floor whilst all the others are being picked up
for washing or a sock is left in the washing machine or
dryer. If the sock is left on the floor you have a good
chance of getting it back and forming a pair. If you leave
it in the washing machine or dryer then you can still get
the sock back, and you will usually find it in the next
load of washing you do. However, if you wash your clothes
at a laundromat, then you are pretty much screwed since the
missing sock will probably piggyback with the next user's
laundry.
A more extreme theory is the Ren and Stimpy postulate that
all socks go to a far away planet in an alternate dimension. In a cartoon called "Black
Hole", Commander Höek and Cadet Stimpy fall through
a black hole, (obviously), and land on a planet with
mutating properties and a mountain of lost socks from all
over the universe. This theory is similar to the answer to
the question "Where do all lost biros go?" as expressed
in the Douglas Adams' The Restaurant At the End of the
Universe. (The biros go to an unknown planet where they
respond to "biro-oriented stimuli" and lead "the biro
equivalent of a good life".) No known being has actually
visited the sock planet or sock dimension as the case may
be, so the theory has yet to be tested.
Steve Taylor suggests there is a sock heaven in a song
of the same name. The song is more metaphorical than
literal, though. Thus, Steve isn't really talking about
socks at all!
So what's the real story? Probably a combination of the
first idea (socks left in the washing machine or on the
floor) and JennyPie's great explanation of how socks are
sucked from the washer during the spin cycle and into the city's water treatment facilities.