Sergey Brin has sold almost all of his Google stock in the past few months

Not exactly. He still owns 19,780,472 shares of Class B stock according to the form filed: https://www.sec.gov/Archives/edgar/data/1295032/000112760216...

Bloomberg profiled Brin's family office last year[0]:

> Running Sergey Brin’s family affairs is a full-time job—and it takes dozens of people. The Google co-founder, who’s worth about $30 billion, has ex-bankers and philanthropy experts working at his family office, Bayshore Global Management. Brin also has employed a former Navy SEAL for security, a yacht captain, a fitness coordinator, a photographer, and an archivist, according to profiles on LinkedIn.

Forbes had a similar exposé on one of Zuckerberg's financial advisers, Divesh Makan[1]:

> Makan shuns the title of wealth manager, according to those who have worked with him. He sees himself instead as part do-it-all financial consigliere, part deal broker/venture capitalist and part human Rolodex. Makan’s currency is access and relevance. While Silicon Valley stalwarts like Sequoia and Benchmark can point to a long list of successful IPOs and a deep bench of management experts, he hawks the “Zuck & Friends” brand to piece together deals between young startup entrepreneurs and billionaire sugar daddies, as well as between the billionaires themselves.

0: http://www.bloomberg.com/news/articles/2015-04-16/google-ser...

1: http://www.forbes.com/sites/briansolomon/2014/12/01/the-spid...

OP, did you really think when you posted this that Brin only held about 25 million dollars in Google?

It makes sense to diversify his wealth.