On Tuesday night, during a dinner conversation about the growth of Facebook and the potential of Twitter, I tossed out a phrase that I can”t get out of my head these past three days at Fortune Brainstorm Tech in California. The conversation was with a dozen or so Silicon Valley execs, from companies like Google and eBay and Palm , and they were noting that Twitter hasn’t invested much in talent–given its global prominence as a communications platform and its need to become a sustainable profit-making company.

So what, it doesn’t matter, said one smart executive at the dinner, contending that “virality” will propel Twitter’s success, at least for a while. “Virality is virility,” I replied.

These last three days have fortified the truth of that statement. Facebook is worth billions of dollars–to private investors–not because of its revenues, which are far from Fortune 500-sized, but because it just surpassed 250 million active users. That’s right, 250 million people use Facebook every month. The social network has gathered 50 million mobile users–particularly valuable because, as a Facebook exec told me yesterday, dual users (who use Facebook on both computer and mobile device) spend twice as much time on the site as single-platform users.

Virality is virility. Yesterday, I wrote about Gina Bianchini, the CEO of Ning, a two-year-old company that helps people build their own social networks–mini-Facebooks, if you will. Word-of-mouth promotion has enabled Ning to collect 29.3 million users and a $750 million valuation. With virality, by the way, comes a need for flexibility–to rapidly respond to customer needs. Ning is launching a gifting service this fall because users told her, “We want this,” Bianchini says. “It’s a co-creation process.”

As for Twitter, we’ll see how its virality pays off long term. Yesterday at Brainstorm, we heard from co-founder Biz Stone, who said that they’ve chosen to stay small–55 employees currently–until they figure out who they want to be. He and co-founder Evan Williams, Twitter’s CEO, don’t even have a sales team. “What are we going to do–stop working on keeping the site up for these three guys to start selling ads?” Stone said. Even without anyone out there selling ads, he said, “the plan is to show some signs of life” this year. Signs of life? That means revenues.

Until those revenues arrive, along with a foreseeable path to profitability, Twitter’s virility–its power and fame and ability to attract capital–will be about virality. Speaking of that, before Brainstorm Tech ended today, my boss, Fortune managing editor Andy Serwer, interviewed Ashton Kutcher, the king of Tweeters, on stage. The hunky actor, who has 2.9 million followers, said, “I shut down a website everyday because I send too much traffic from my Twitter feed.”

That’s viral virility.