I disagree with this assertion:

Since the invention of Facebook and Twitter, the brain size factors that limit the size of our potential network size are easily circumvented.

I would argue that social media eliminates the range of our network, but it doesn't do anything for the size. There's no reason why our brains should have any more of a handle on large numbers of contacts with Facebook than it should with an address book and those have existed for hundreds of years. Keeping contact with people beyond our immediate social network is what weddings, christmas cards, funerals and all the other social rituals we've developed since anthropology was possible. They're a lot easier to interact with in the digital era, sure, but our brains are no better or worse at bouncing amongst 900 people now than they were in the neolithic era. Just because you have a thousand Facebook friends does not mean that you, right now, have a viable social relationship with 1000 people.

The other problem with mourning people we don't know is that our representations of them are only partially "them." Everyone reading my writing has an idea of who I am, but it's formed from the off-handed observations of an extremely-limited contextual dataset. I'll guess that roughly a third of the people reading this would have believed that I can use the phrase "contextual dataset" in conversation, in person, but I can and do - so now you have a datapoint. You do not know that I tend to flick my bracelet around my wrist as I think or that I talk with my hands a lot. You certainly don't know how I hold your gaze as you speak.

Which isn't to say you don't have that impression of me - but it isn't me it's an amalgam of everyone your brain thinks might match my paradigm. Thing of it is, if we all got together in a room and chatted over a shared experience, we'd know each other because we'd have the chance to observe non-contextual cues, assess body language, listen to tone of voice and observe each other in a rich environment.

So when someone we met at a bar once dies, we feel something and we know it to be genuine. But when someone we've only interacted with via typing dies, we feel weird because from our perspective, only a tiny percentage of that person actually died. Our avatar of them - our representation - remains like a husk, haunting us... and we know that we have now lost any opportunity of reconciling truth and fiction.