Microsoft co-founder Bill Gates has been named as the wealthiest person in the US for the 17th year in a row.

Forbes magazine put his fortune at $54bn (£34.5bn), with investment guru Warren Buffet second with $45bn.

Software tycoon Larry Ellison was in third place, while Wal-Mart heir Christy Walton was fourth on Forbes' list of the 400 richest Americans.

It took a net worth of at least $1bn to earn a spot in the rankings - up from $950m in 2009.

The collective net worth of the 400 billionaires rose by 8% from 2009, totalling $1.37 trillion - approximately the same as the GDP of Spain or Canada.

Social network trend

Those gaining wealth in the list included Facebook co-founder Mark Zuckerberg, whose wealth grew by 245% to $6.4bn, the report said, putting him at 35 in the list.

His fellow co-founder Dustin Moskovitz is the youngest to make the top 400. The 26-year-old is eight days younger than Mr Zuckerberg and one of 16 new entrants to the list, which also includes another Facebook figure, Eduardo Saverin.

Another gainer was Manchester United owner Malcolm Glazer and family, while among those losing ground were the family behind confectionery giant Mars.

And there were 34 dropping off the list, including real estate mogul Tamir Sapir.

The 18 people returning to the list after a period of absence included Sidney Kimmel, at number 365, who made his fortune in the clothes business before producing films including The Kite Runner.

The report also found that among the 400 members of the list, 231 had donated to the Democratic Party between 2006 and 2010 - giving a total of $6.2bn. This compared with the 247 people who donated $7.3bn between them to the Republican Party.