If we're talking written letters, most of these offices at least have staff that read them. I've sorta lobbied before and they don't get quite as much mail as you imagine.
And they respond more often than you'd think. Your attitude is pretty prevalent so the chance to write back and change a voter's impression is hard to pass up.
By the way I also know for a fact that Scott Wiener's staff reads San Francisco's subreddit. Commenting about Wiener's vote in there will reach his staff. Given the bay area presence here it's also possible somebody "who knows a guy" will even see this thread name-drop him.
And they respond more often than you'd think. Your attitude is pretty prevalent so the chance to write back and change a voter's impression is hard to pass up.