These ARE social networks, and they're WIRED, and have a lot of structure at the local level where, as O'Neil instructs us, is where all politics is anyways. Any up-start "political social network" that ignores that existing infrastructure is like Facebook ignoring colleges.
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.
The shortage is not of opinions, but attention. Why should I or anyone pay attention to your site? I can write my political opinions anywhere: Obama sucks. See? Look how easy that was.
I'm a voter. Why should I use your site? No reason.
I'm a politician. Why should I use your site? No reason.
You need to rethink. Go talk to some actual political people. No one has any reason to use your site or to give you any money. Most importantly, there's no virality factor - no one has any reason to try to get their friends to use it.
I just went to your site. It asked me something about a bike lane in Madison Wisconsin. I don't have the slightest idea what it is talking about and have never been to Madison Wisconsin. I voted no.
Why should a voter use such a website? Because his/her humble click on a Support/Oppose button could aggregate with many others' clicks and make a difference when the number of people expressing an opinion is high enough to get the attention of the politicians and of the media. The random expression of an opinion on the street influences just the few listeners that happen to be arround.
So yeah, I think it's broken, it barely ever worked well.
And in the context of this article, I think we do a ridiculously poor job of listening to constituents, largely because there isn't a successful near-universal political social network that forces politicians to listen.