According to the 2015 Forbes ranking of the wealthiest people in the world, San Francisco is home to 18 billionaires, tying it with Mumbai for the eighth most for any city. It turns out that several billionaires associated with San Francisco don't actually live within the city limits, making it somewhat tricky to define who should and shouldn't be counted.

For example, Mark Zuckerberg, co-founder of Facebook worth $35 billion, actually resides 35 miles south of San Francisco in Palo Alto. So does Larry Page from Google. Oracle tycoon Larry Ellison really lives in Woodside. Even though these are all in the greater San Francisco Bay Area, the city itself is home to a (relatively) less wealthy crop of billionaires.

Like most of the rest of the California super wealthy, billionaires in San Francisco tend to be younger and more technology-oriented than billionaires in more traditional hubs, such as New York, Tokyo, Moscow or London.

1. Dustin Moskovitz ($7.9 Billion Net Worth)

Dustin Moskovitz is best known as the former Harvard roommate of Mark Zuckerberg. After helping Zuckerberg launch Facebook, Moskovitz stayed in school for two more years before leaving for Palo Alto to develop the social media site.

In 2008, Moskovitz left Facebook and started his own software firm, Asana, which focuses on task management and team communications for businesses. Even though Asana has been a successful company, most of Moskovitz's net worth is tied to his Facebook stock. He also helped found Good Ventures, a philanthropic foundation, with wife and Wall Street Journal reporter Cari Tuna.

2. Travis Kalanick ($5.3 Billion Net Worth)

Few billionaires' stock is on the rise as quickly as Travis Kalanick, the chief executive officer (CEO) of Uber Technologies. The former UCLA dropout is a classic example of entrepreneurial insight and drive; his first startup was sued by the Motion Picture Association of America and went bankrupt, and his second venture, Red Swoosh, was bought out for more than $18 million worth of Akamai Technology stock. Kalanick used this money to help launch Uber, which has since become a worldwide sensation.

Uber has been valued at more than $50 billion and threatens to drive the antiquated city taxi service into extinction – the only cab companies successfully competing with Uber are those that have petitioned local governments to place restrictions on the private car company.

3. Marc Benioff ($3.4 Billion Net Worth)

Marc Benioff is the chairman and CEO of Salesforce.com, a cloud software firm and customer service platform. His rise to success is a mix on innovation and fortunate circumstance; Benioff was the right-hand man and protégé of Oracle's Larry Ellison for 13 years before co-founding Salesforce.com in 1999.

According to Forbes, Benioff owned 5% of Salesforce.com and 13% of Fitbit (a health tracking service) as of 2015. Even though Salesforce.com has never reported a profit and failed to close a megadeal with Microsoft in 2015, Benioff's shares still command a market value greater than $3 billion.

4. Doris Fisher ($2.9 Billion Net Worth)

Doris Fisher is one of the wealthiest self-made women in history. One of the co-founders of the retail clothing chain Gap (along with husband Donald) in 1969, Doris has served as the company's merchandiser and sat on its board of directors.

Today, The Gap runs more than 3,000 stores and is in charge of several prominent clothing brands, including Banana Republic and Old Navy.

Fisher is an influential proponent of charter school education. She and her husband founded the Knowledge Is Power Program (KIPP) and have donated more than $80 million to the cause of improving urban education. The couple has traditionally focused on minority and single-parent children in major cities, with particular emphasis on recruiting talented teachers to otherwise undesirable locations.