The number of Americans using Twitter dropped 8 percent in October from September, marking the second monthly decline for the social-networking site this year, according to research firm ComScore Inc.

Twitter Inc., the No. 3 social-networking site in the US, had 19.2 million users in October, ComScore said yesterday.

The company had growth of less than 1 percent in September and declined in August.

October’s number was still up more than 13-fold from the year-earlier period.

The month-to-month decline contrasted with a 2 percent increase for users of Facebook , the most popular social network. News Corp.’s MySpace, which ranks second among US social-networking users, fell 2.5 percent from the previous month. The Post is owned by News Corp.