This is a new word to me but it seems to be catching on and works well metaphorically. In the online sense it’s already in MW, etc.
Word Spy finds a cite from 1993.
Also here and here.

It’s very familiar to me, but perhaps I spend more time in online communities than you do.

Discussed in this thread on the old board. As usual, wordgeek had the wrong end of the stick and imagined it was something sexual.

seems to be catching on

I agree with LH. It was a well known term of art on IRC and Usenet back in the 90’s.

Yes, very familiar to me too. It’s frequently shortened to sock.

I might as well add my voice to the chorus: this has been a common term for longer than I’ve been on the internet (some 16 years now).