Here are volumes of mail-sent-per-adult for three categories that make up over 90 percent of the volume of what is delivered by the USPS: single-piece first class mail, first-class mail presorted, and marketing mail.
Single-piece first-class mail per adult started dropping in 1996, and has fallen by 70% since then.
First-class mail in the presorted category (which is more likely to be mailings sent by firms or government to consumers) continued to rise up until the Great Recession, but has declined by about one-third since then. .
Marketing mail dropped in the Great Recession, and is now down by more than one-quarter from 2007 levels, but its decline has been much smaller in recent years. As the report notes: "Marketing Mail is also playing an increasingly prominent role in the Postal Service’s product portfolio. At approximately 80 billion pieces, Marketing Mail volume is higher than FCM-SP and FCM-Presort combined. In 2015, it made up about 52 percent of total mail volume."
But in addition, the report notes that the rate of decline in mail use seems to have slowed in the last few years. I've written in the past about steps that the US Postal Service could take to improve its financial outlook, and I won't repeat that here. After all, the possibilities for innovative change at the Postal Service have been strangled by political infighting which led to a situation in which, by the end of 2016 and since then, none of the nine appointed slots on the Board of Governors of the Postal Service are filled.