Is the shine beginning to fade for Google+?

Google's social-networking foray saw fewer U.S. visitors last week, and those who did visit spent less time on the site on average, according to new data released today by market researcher Experian Hitwise.

Google+ had 1.79 million visits for the week ending July 23, a decrease of 3 percent compared with the previous week when the network had 1.86 million visits, Experian Hitwise reported.

The average visit was 10 percent shorter, down from 5 minutes 50 seconds to 5 minutes 15 seconds.

Google declined to comment on specifics of the report but cautioned it's based on the number of visitors and not the number of actual users. Google also said third-party metrics don't factor in two "very critical" modes of interaction, namely mobile usage and navigation bar usage, in which members interact with Google+ from another Google product, such as the drop-down menu while in their Gmail account.

Google+, the Web giant's latest attempt to challenge Facebook for the social-networking crown, launched late last month in a field trial, letting in only a handful of users, who could later invite their friends and family.

Despite this, Google+'s early growth has been meteoric, hitting the 20 million member mark in its first three weeks, according to data released last week by market researcher ComScore. If that data is accurate, Google would have doubled its membership in just one week. Google CEO Larry Page reported on July 14 that the social network had reached 10 million visitors and had received a "ton of activity" with more than 1 billion items shared and received each day.

Updated at 9:10 p.m. PT with Google cautioning about the report's data.