And, it's kind of funny to claim that it is the left socks which get lost, because socks are symmetrical, right? So, you really can't tell which are left and which are right until you choose which foot to put them on.
However, a few months ago, I bought 18 pairs of socks which are not symmetrical. They are polo socks** and the left and right socks are distinguishable. Here is a picture of my right ankle wearing a right polo sock.
The little embroidered picture of a mounted polo player is circled in red.
After several months of wearing, laundering, sorting, wearing, etc. it turns out that three of the socks are indeed missing. And, they are all left socks.
Only one data point, and hardly a scientific experiment, but there you have it. Left socks get lost.
[Added 2019/02/13: *the link for "socks get lost" is now a dead link, but I found the page on archive.org (missing its embedded images, but still) and you can visit is here. And **the link for "polo socks" is also a dead link, but you can visit the archived page here.]
