How old is your oldest pair of socks? I tend to hold on to mine until the bitter end. They inevitably get a hole in the heel and then I have to admit defeat and chuck them out. Handknitted socks are another matter though. Even then they don't get the chop.
I was sorting out the laundry and realised that a pair of my husband's handknitted socks had developed a hole. Thanks to my back catalogue of posts I can see that I finished these socks at the end of 2010 and they have been going strong since then. Not bad for socks that have been in regular use. I bought a darning mushroom from a charity shop a few years back and it has been put to good use.
I'm afraid I can't give you a masterclass on how to darn socks as I am fairly slapdash. I just sew sort of ladders back and forth across the hole and then weave the yarn up and down these ladders to fill in the gaps.
I think it's supposed to look like you haven't needed to repair it at all, which is not quite how it looks when I do it - but, well, it looks better than before!
The socks live to fight another day!