To match pairs, I sew a wee bit of cotton into the toe of each sock. A different colours for each pair. Makes matching them after washing so much easier.
I often wear mismatched socks by choice so as long as it has a bed mate of a similar fabric then no, but if it's a real one off texture wise then they go in the bin :)
Same as kval. I just throw away the one with the hole. I only ever wear socks outside the house if I'm wearing boots. So as long as they're the same fabric they're fine.
alba, Mrs. bookbinder says the same as you; darning wool isn't as easy to find in the shops as it used to be, thanks to this throwaway society that we live in now.
I used to find it relaxing, sitting watching something on tv and darning.
I remember my mum having quite a selection of darning wool on the cardboard thingies :-)