The Verge reported that as of Tuesday, the first day that the site went live, COVIDtests.gov had 685,817 visitors at about 1:30 p.m. Eastern. All other .gov sites combined only had 419,615 visitors at the same time. Given that the site went live a day ahead of schedule, it’s probably safe to assume that those numbers didn’t decrease in the following days.
As of about 11 a.m. Friday they did start to go down, with the USPS site for placing the orders edging out COVIDtests.gov for the top spot. However, COVIDtests.gov still sat in first place for top .gov domains visited over the previous seven days.
You can see the results for yourself in real time here.