Today’s news that Ning CEO Gina Bianchini is leaving the company is a surprise to many in Silicon Valley. Even to those who know her.

I’ve been surveying tech folks about the announcement that she’s handing her CEO role to Jason Rosenthal, Ning’s COO and head of business operations since late 2007. And although Ning, which Bianchini founded with Marc Andreessen, has had churn and reported discontent among staffers recently, few saw this mega change coming.

Bianchini’s exit is mega because she was the face of Ning. Not that the bold, bald presence of Andreessen, the serial entrepreneur who is Ning’s chairman, isn’t effective in promoting the start-up, which helps people build their own social networks online. (As Home Depot is to homeowners, Ning is to web inhabitants who seek to belong to niche networks relating to their interests and passions.) Ning has been Bianchini’s passion since 2004.

A native of Silicon Valley who began her career at Goldman Sachs in New York and then returned, she has waved the flag for Ning tirelessly. 

She appeared on the cover of Fortune in 2008 (that’s her on the left), as well as Fast Company. Last week, Bianchini, 37, talked about the power of Ning on Ning on Charlie Rose. This past weekend’s WSJ magazine featured a glowing spread on Bianchini and her workout routine. And this week, Ning has a major presence at the tech-centric South by Southwest Film Conference and Festival in Austin.

Ning claims to have more than 45 million registered users–healthy growth from 29.3 million last July when Bianchini filled us in on her last round of money-raising–at an impressive company valuation of $750 million. (Ning has raised some $120 million.) But blogging start-ups like Tumblr and Posterous and location-based web services like Gowalla.com and Foursquare (which are all tied to users’ interests and passions) are cutting into Ning’s buzz factor and market opportunity. Moreover, as Facebook expands far and wide, Bianchini’s absence will likely hamper Ning’s brand-building.

Her silence raises question. She didn’t Tweet today about her departure–odd since she typically broadcasts multiple times daily on Twitter. She also declined to talk about her departure, replying to me in an email this morning:

“I’m leaving Ning in very capable hands and have total confidence in the team and the vision that we’ve worked for 5 years to build into a real company. Now I’m excited to look towards my next adventure, of which I don’t know what it is yet :-)”

We know this much: Bianchini is becoming an Executive In Residence (EIR) at VC firm Andreessen Horowitz, as Andreessen explained in a blog post this morning. He wrote: “My venture capital partner Ben Horowitz and I are looking forward to being able to benefit from her insight and wisdom even more than before.”