Look at people who get addicted to MMOs like World of Warcraft for example. They voluntarily limit themselves to the messages they receive from the game and this influences their behavior significantly.
The proof that control of these messages and what message are received by people is an extremely valuable commodity is that advertising is a multi-trillion dollar industry.
Even if everything was perfectly truthful, there is only a small amount of time for people to digest and absorb the world with their limited perception. Thus, the control of which limited set of messages that people receive is also a huge source of power and why a commercial in the super bowl is worth more than a random banner ad on some no-name website.
Reddit, Twitter and most social media are attempts to optimize this messaging problem.