NBC’s local websites for its 10 owned and operated stations have effectively lowered the average age of users from 44 to 33, boosted the average time spent on site to 12 minutes, and grown its user base to 10 million unique visitors, according to NBC San Diego Director of Integrated Media Sean Monzet, who was speaking at San Diego Interactive Day yesterday.

Monzet says that NBC’s big strategic move a couple of years ago was to remake its sites from TV station promotional websites to NBC city sites, reflecting a broader mission. A number of verticals have since been launched, including The Feast (a dining/lifestyle site), The Goods (a group buying site launched last month) and The 20 (top special interest stories).

Local properties also are encouraged to build their own, unique content franchises. Soundiego, a local music site, was launched in early 2010 and features six bloggers. It is updated three to five times a day and is San Diego’s largest local music site, says Monzet. Other local verticals include PropZero (a politics site), a site for the San Diego Padres and a travel destination site called “Worth the Drive.”

Social media has played a big role in NBC’s growth. Twitter followers are up 300 percent and Facebook friends are up 100 percent since July 2010. “Everyone is on Twitter,” notes Monzet, who adds that staff members are awarded karate style “belts“ ranging from white to black for their prowess on Twitter. Each of the stations’ vertical sites has its own social media account, he adds.

On the hyperlocal front, the San Diego station has partnered with The Voice of San Diego, a local nonprofit news organization. The two organizations now produce a couple of weekly segments, and are working to cover specific stories and topics, says Monzet. The arrangement has gotten national attention. Under terms of the FCC’s agreement earlier this year allowing NBC and Comcast to merge, every NBC owned and operated station is required to develop similar arrangements.