The public's expectations of how they interact with elected officials has changed dramatically over the last few decades, but many offices haven't changed their practices to match. Now that everyone has access to many sources on congressional activity (some reliable, some not), people no longer rely on congressional offices to explain bills and provide them with legislative updates. Instead constituents want to hear how the Representative/Senator feels about issues that are important to them.
Another dramatic difference is the rise of issue campaigns generated by advocacy organizations. For most offices, this type of message now accounts for 75%-85% of incoming mail. This begs the question: Do constituents want or expect a full response or are they just making their position known to the Senator/Representative? In looking at a few offices' email open rates (available in most correspondence management systems), it seems that only about half of the recipients open the office's reply. Constituents want to know they've been heard. The key is to write content that assures them they have been.
CMF asked one of the offices that made our recommended changes to their mail system if their Member had noticed the changes. The staffer replied, "Yes, because constituents noticed and told him! They commented that they feel like they know where he stands and have even forwarded messages to their friends." She also indicated that the office's greatest success with the changes was that they were able to keep up with the increased communications in 2017! They produced higher-quality letters in less time, so they responded to many more messages with less staff time involved, even during the peak of early-2017 communications volumes.

Many congressional offices are writing letters like it's 1979, but that's not what constituents expect in the current 140-character-limit world. Offices should focus more on connecting with constituents in short messages and less on trying to persuade them with legislative research or impress them with congressional letterhead and Member signatures.
CMF thanks Democracy Fund for funding the Congress 3.0 project and the Resilient Democracy Coalition.