A little Googling seem to suggest that this figure is somewhat representative: the US has slightly more mobility than the UK, which have less mobility than most of Europe:
Wait a second. That data just measures correlation between father and son in terms of income. Thinking through it, if you had more sons do worse than their fathers economically, they would show up better in this data.
It's not measuring upward mobility, but mobility in general. Seems suspect.
The rich stay rich, more than the poor stay poor (in Canada).
But the number is about [relative change in income across] the population - if most people are poor (which they are) high mobility across the population likely means poor people are moving up (or indeed getting much poorer - halving your income when you're already in the lower quartile would probably be as big a shift as doubling it - if not more).
http://www.epi.org/publication/usa-lags-peer-countries-mobil...