Dobby wasn't in Dumbledore's employment yet in "Philosopher's stone". Dobby didn't get free from the Malfoys until "Chamber of Secrets", and he ended up at Hogwarts some time after that, when he couldn't get paid work anywhere else. So he couldn't have given Dumbledore anything yet.

But that's really beside the point anyway, because we now know from "Deathly Hallows", that Dumbledore never saw any socks in the mirror, but rather something about his family.

We know what it is, but I would not have known beforehand. If I had just read the first one, I wouldn't have known, and I doubt anyone else did either.

It's really interesting though, that it wasn't mentioned again until "Deathly Hallows".

Yep. I think that that was Rowlings plan all along,to make us put it in the back of our head.

His family.

It was said in the last book that he saw his family together again (I think) but I'm not sure...