While the traffic to the social media giant's mobile app has grown, it has not been enough to offset this fall, analyst Stephen Kraus, the chief of insights at SimilarWeb, said.
However, the Facebook network — which includes Instagram and Messenger — continues to grow.
"Users of the Instagram Android app, for example, now average 56 minutes a day on the app, up from 27 minutes a year ago," said Kraus.
"This transition reflects how Facebook is focused, not just on growth for their main site, but rather on expanding their entire ecosystem."
Facebook lost about US$120 billion of its value on 26 July after it announced what was its slowest growth of users and warned that revenue would fall for the remainder of the year. Founder Mark Zuckerberg's personal fortune fell by almost US$16 billion.
For many years, the five sites attracting the most traffic in the US have been Google, Facebook, YouTube, Yahoo! and Amazon in that order.
But that is about to change, as SimilarWeb's projections showed that YouTube would displace Facebook to become number two and Amazon would displace Yahoo! to take the fourth spot as it did in July.
Google is far ahead of the rest in terms of visits to its site, registering 15.2 billion visits in July. This was despite a fall in traffic to the website itself, with more people using the mobile app to access the search engine.
None of the other four had more than five billion visits per month, with July figures showing Facebook at 4.7 billion, YouTube at 4.5 billion, Amazon at 2.08 billion and Yahoo! at 1.9 billion.
Regarding Amazon, Kraus said while the other four sites in the top five got most of their revenue from advertising, Amazon obtained about three-quarters of its revenue from transactions.
"Given that transaction-oriented sites generally need less traffic than ad-supported ones to be financially successful, Amazon’s traffic of nearly two billion monthly pageviews and its industry-leading conversion rate combine to highlight the site’s tremendous success," Kraus said.
"Amazon’s monthly traffic first surpassed Yahoo! in the holiday season of 2017, and again in July 2018 (spurred by the success of Prime Day). If current trends continue, Amazon will soon begin generating more traffic than Yahoo on a regular basis."