They might be also mimicking each other, consciously or not, in an attempt to show identity. As in: "I identify myself as a rich person, so I'm going to write like a rich person". It's a form of group bonding, much like kids with their six seven.
Note this does not contradict Tannen's hypotheses โ on the contrary, I think all three things (illusions of importance, fake friendliness, and group bonding) are deeply integrated. Specially because the middle class is often damn proud of their spelling, as the ability to follow the spelling standards is seen as a proxy for being cultured.