I would assume that they do some market research (or take someone else's market research) and get some conclusions from that study. Some market research can be done asking people (whether they use different polls throughout the year, or doing a proper survey, that's something to see)
Some other data is given by the communication companies themselves because they can measure how and when you connect, how long you stay connected, if you're using a mobile or not and if it is a data connection or a voice one.
ISP also provide their own statistics about use. Of course, even if every site is different and its public may have nothing to do with your neighbours' site, your own server statistics may provide some light too.
So if a Telephone company says that they have measured that their antenas for mobiles have been used a 30% more than the last year, and that 75% of that use is related to data transfer, you can safely say that there's been an increase of mobile using data services which 90% of the times are web related and it is more or less safe to assume that the use of internet during the next year will increase even if at lower rate.