ComScore has released its list of the top fifty most-visited websites in the U.S. for September, and, for the first time, Pinterest has made an appearance, albeit barely.

Pinterest’s 25.3 million visitors in September was good enough for 50th place on the list, behind its more senior social media brethren MySpace (47th), Instagram (44th), Twitter (27th), LinkedIn (26th) and Facebook (4th).

It’s worth noting that this data is estimated and only reflects visits to the websites of these social networks (i.e., Facebook.com, Twitter.com) and does not take into account traffic from third-party apps and mobile phones, which would likely significantly boost the placings of all these channels, but especially Facebook and Twitter.

Google’s collective empire of sites heads the table, ahead of Microsoft and Yahoo.

(Source: ComScore. Hat tip: CNet.)