San Francisco is experiencing another explosion in startups, and one of the best ways to visualize that is at at the local tech community's hottest recurring event, which is the "startup mixer" held by SF Beta every other month at 111 Minna.

SF Beta is the brainchild of Christian Perry, now 26. He launched it when he first moved here from Chicago after finishing college back in 2006.  Perry notes that his meetups are attracting record crowds these days.

"There is a big spike this year, up to 650-700 attendees, or more than twice as many as in the past," says Perry.

Almost all of that growth, he says, is due to the rise of social networking companies like Twitter, mobile app developers, and new advertising startups, which have catapulted San Francisco into the new center of tech innovation -- more than anyplace else, including Silicon Valley.

Perry estimates that there are currently between 4,000 and 6,000 startups in the City, if you define "startup" as a company that has at least one round of angel funding so that the founders have been able to quit their day jobs. SF Beta's current mailing list sits at 4,800.

Plus, countless other innovative ventures are in the works, which bodes well for SF Beta, as well as for the local economy.

The SF Beta events are unique in that more than 60 percent of the attendees are founders or executives of a startup. Millionaires (and millionaires-to-be) are a dime a dozen. Most are young, and 75 percent are men. Although they are social events, for the most part these are gatherings of entrepreneurs connecting with one another, "not a dating event, or a singles night," notes Perry.

Back in the beginning, Perry's first meetup in September 2006 attracted 35 people; the second 85, and the third event 165 attendees. Perry estimates that back then there were between 1,000-2,000 startups inside San Francisco's city limits.

Each mixer has a theme.

"I like to pick themes that represent a topic or a space that is gaining a lot of traction and growth and change," says Perry. The next one in early March is focused on "maximizing mobile apps."

Around a dozen or so new companies reflecting that theme will set up tables at the events, where attendees can get a hands-on tour of their products and meet their founders. But there are no formal presentations -- "at first we tried that but they didn't work because too many people deep in conversation kept talking right through the presentations," explains Perry.

Now that he has succeeded in establishing SF Beta, Perry is hard at work extending the meetup model to four other cities -- Boulder, Portland, Seattle and Boston, close to where he now lives, in Cambridge. He is currently a Fellow at Creative Commons, which works to extend universal access through copyright licenses free to the public.

Despite having left San Francisco 18 months ago, Perry flies back for every event and maintains a strong network in the City.

"Most of the power and money in technology may still be in Silicon Valley," says Perry. "But the next generation of innovation is happening in San Francisco."