I would guess: Because time spent on Facebook is relaxing, whereas most internet discussions feel like a drunk fist-fight without winners. (Of course, that may be the nature of democracy itself.)
In the U.S. at least, the Internet plays a very large and growing role in elections. In fact I would argue that in the most recent election, the Internet scored a decisive win over TV ads--the most effective political tool for the past 40 years.
Once officials are in office, they can be petitioned (lobbied), and again the Internet plays a major role in how unions, associations, nonprofits, etc. organize their grassroots to do that.
Keep in mind that "Internet" means more than Web; in the case of politics, email is still king.