Where can I see the actual number of nodes that joined the hard fork, in comparison to the others, which didn't? What does the difficulty tell me about that relation? i.e. 100% : 91% as here
From the blockheight difference you can see that pretty much nobody mines the nonfork chain.
Normally a block is mined every 14 secs, the difference
(Blockheightprofork - Blockheightnonfork) * 14s = Time ahead
It's over 1h and gives you an idea that currently nobody mines the Nonfork chain.
Also Blockheightnonfork is currently 48 * 14s is arround 11 minutes. So nothing happens on Nonfork...
Another nice indicator is the current hash rate
Highest Avg Hashrate of 4437.5176 GH/s was recorded on Monday, July 18, 2016
And we are quite there again...