An important post by Harman on Latour's concept of plasma:
“Society” for Latour has nothing to do with a social realm as opposed to other kinds of realms. The social for Latour contains absolutely everything, including fictional and nonexistent beings (as long as they have some sort of effect on other things).
The idea that the plasma isn’t inherently unformatted but simply not yet known to humans gives a central status to the human knower that is simply not part of Latour’s outlook.
In short, plasma is one of the most metaphysical concepts in all of Latour’s work. It cannot be tamed or decaffeinated by trying to claim that it’s just the harmless and tepid point that sociologists must humbly realize that they don’t know everything yet.
Instead, as explained in Prince of Networks, Latour needs to posit the plasma because his overidentification of things with their effects on other things leaves him with no way to explain change...
I agree that plasma is a metaphysical concept. But I'm not quite sure of that last point. I'll have to think about it. See Latour, Reassembling the Social, 244-246. The general scheme of Politics of Nature is also relevant: What IS the collective being formed in the large?