It appears to be a hardware limit - humans can only consider a given number of other humans as individuals. Everyone else composes "the others" and are treated less favorably.
In fact that empathy is what allows normal conversations to occur "in the real". That people lose that empathy online isn't surprising, they are talking to a screen, after all. But I also believe that those who can see past the pixels and into the virtual eyes of the person they are talking to make better real-life people. It also makes for much better conversations[2] online.
0. I'm going to go Godwin here just by saying I'm going Godwin here. You know what I mean!
1. My wife works for a pediatrician and has told me they are no longer considering Asperger's as its own syndrome (disease, disorder, I always get those mixed up); it is just a place on the spectrum. I have no separate source and just thought it was interesting. I also realize trusting what a single doctor says is a really stupid move on my part, so please feel free to shred me.
2. If for no other reason than you don't have to read a bunch of self-effacing, defensive crap, such as I wrote in the above footnote.
I guess it makes sense in a way, but it'll probably just confuse popular conception of autism even further. Ah well.
This theory always seemed to me to be attempting to account for the absurd size of our neocortex, and proposes a somewhat convincing rationale for this (i.e. the need to socialise and deal with many other humans).
Now, this sort of selection pressure has not reduced in the modern world. Far from it, one's ability to maintain larger social groups could easily be extremely adaptive in that you have both a greater opportunity to meet potential mates, and may additionally come into contact with resources through these larger groups which make you a better potential mate.
Therefore, it would seem to me that while Dunbar's limit may be at a particular level now, in our current society it may well be being selected for, and perhaps will be much higher in the future. Or perhaps I am completely wrong.