I've encountered many, many other fliers, whether on their laptops or smartphones, checking/composing e-mails during flight. However, that's all they're doing. They don't actually send/receive new ones until they land.
They could also be playing games like wildcatlh said.
I have checked in for a future flight while in the air (yes, that's a no no), but that was shortly after takeoff. I got brief periods of connectivity according to my phone, but I don't think I would have been able to connect. No idea how you could stay connected for 2+ hours on a regular flight w/out wifi.