Cant say I know ANY billionaires personally.
But of the millionaires I know:
Many made their cash through being professionals, and investing in property.
One is an options trader, and now is worth $7million, the majority of this came through real estate invesment (though he does earn $.35 million per annum), he has not made his money by investing in options himself (ironic?). He is 34 and married.
One is an orthadontist (not sure the spelling), and so is his wife, between them, they earn nearly $1 million per year, and now have two practices. They are worth around $5 million. They are 34 and married.
My brother opened a fast food franchise (started the research a couple of years ago) and now owns the beginning of a franchise. He is worth $2 million. He is 26.
The rest I know are old, safe, professionals, who earn their pay packet, support their kids, and buy property every now and then. These couples would each be worth about $2 million. They are all around their 50s.
I myself, am in my mid-twenties and have just got underway. Though I did trade stocks since I was 12 years old. So far I have around $100,000 in cash and liquid stocks. I am currently a financial analyst with 2 degrees.
So there you go, nearly all the millionaires I know (and none are overly rich, yet), made their money through first being professionals and saving, then investing in property. Other than my brother, who borrowed, started his company, and is doing the best of them all for his age.
I know plenty of other extremelly wealthy people, but I dont know them personally enough to estimate their wealth. One is the CEO of Computer Associates, I know what he earns, but he blows it damn quick also on his lifestyle!
The only part about property I dont like, is that at least here in Australia, there was a baby boomers generation, these baby boomers are all the ones who are in the high paying jobs now, and who are driving up the property boom. With them ageing, the ageing population is going to dramatically increase once they hit retirement age (in the next several years). Who is going to fund their retirements? Certainly not tax reform, it will have to be funded by themselves. Which means, the sale of hundreds of thousands of investment properties, ultimately pushing down the return on property. This is my theory, and a theory taken by many financial planners and advisers in Australia currently. One I know just sold 3 houses himself in the past year, as he beleives the crash will hit in the next few years. Should be very very interesting, the economy has NEVER seen a similar situation.