O.k., I have always pondered this question, and I still can't answer it! However, I decided not to feed the sock monster anymore and take charge of the situation.
When we were little my mom had a brilliant plan of not loosing any more socks. She bought a large package of wooden clothes pins and pinned the top of the socks together, so she couldn't loose anymore socks. Brilliant I say...Brilliant!
I taught my kids to do the same thing. As soon as they dirtied a pair of socks, they either needed to roll the socks together (so I could add the pin, just before washing them) or they could find a pin themselves. Believe me, this plan has worked wonderfully! Now we all have mated socks!
(We just need to find a cure for the holes in our socks (just kidding). Actually, we buy new socks every time conference rolls round (every 6 months)).