From a redditor who works in the senate
The vast majority of people who contact their elected representatives are raging idiots. I've been to townhall meetings where people shout at the Senator, rant and rave about this and that. I've answered constituent mail... 90% of it is garbage. Form letters, petitions is half of it... another load is ranting from crazy people. Calling us communists, ranting about the President, totally uninformed whacko garbage. It's gotten worse with email... most emails are just all caps, no punctuation rants. And then there's always the conspiracy stuff... FBI is reading my mind, etc.
That said- there are many well written and informed constituent letters. And they all get read and get a response. But when one person writes about an issue, it's a drop in the bucket of crazy. Things still get done though, there have been cases where constituents brought to the attention of us things we didn't know, and we made policy decisions based on that.
Organization and numbers helps. Form letters are ignored, I recycle them immediately. But take the SOPA/PIPA debate. Thousands of informed people wrote about their concerns. The letters were well written, polite, informative, and short. There were so many we're still answering them. So it can make a difference, but form letters and crazy ranting make it hard to stand out.
I choose to believe the smart people are too busy/disinterested to write. The other option is to accept that most Americans are crazy uninformed lunatics.
And yes, we do listen to lobbyists and advocacy groups. Why? It's not because they meet with us in a dark room and promise us money. It's because lobbyists are also policy experts who know their shit. Yes, I did pay attention when the nurses lobbying group came in. Why? Because they showed me studies from the New England Journal of Medicine supporting their position, they highlighted a specific piece of legislation, and they showed me where it has worked in the past. They made a compelling case. The average citizen can't seem to form a correct sentence.
I'm sure this will be buried, but that's the truth nobody can ever admit on Capitol Hill. And it's an unpleasant truth, but it's the truth. It falls on you, the citizen, to change it.