I think .Net was probably 'the first dot com'.
There were six TLDs to begin with, of which three were available to the public from 1985, being .com for commercial use, .net for networking, and .org for non-commercial use. The other three - .mil, .gov and .edu - were and still are restricted.
Because the world wide web had not been invented in 1985, most computer companies used .Net to network computers, so of the three publicly available spaces, this extension would most likely have been the most popular.
I imagine that most people in 1985 would have been asking what can we do with a dot com?
My school (not in the US) had a couple of networked computers when I was in my final year, and I'm pretty sure they were running on the .net space
Anyway, my first pick after a .COM would be .NET