I would say that they are reading fewer books but I think total number of hours reading is similar or growing.

Reading tweets and text messages is still reading. My nephew has trouble learning to read, until he started playing minecraft and needed to read websites and instructions for mods and such. Then getting his first cellphone did away with any concept of reading difficulties. We have entire economies of people reading text on computers all day (ie my job). I would bet that the average person today read better/faster than their equivalent in centuries past. They are reading junk, but they are actually reading.

A lot of the time they're skimming, rather than reading in depth. This is great for picking up key information quickly from a wiki, but it's also easy to miss something complex-but-vital.