You must take into account that the link Gnasher has put up is from 2011 when the issue was properly being addressed full on, but also that you're the first person in quite a few years now that have brought this problem up on here.
After pages and pages of discussions, some got the shaft replaced (courtesy of Yamaha), others used extra strong Loctite, spot welds, etc.
But although still not perfect by any means, because it was primarily a shaft thread issue, the main thing that calmed down more reports of it happening had to be the replacing of the OEM 9mm sprocket nut with the newer 12mm one.
Obviously that doesn't apply to 100% of bikes by any means. It would seem that at least the 98 (maybe 99) versions were unaffected, and others simply haven't had the issue at all.
On my bike the 9mm nut fused itself onto the shaft, rather than come loose, so was a complete bugger to get off when i had my sprockets and chain changed.
My point though is that i wouldn't let it put you off buying another Fazer by any means. You've been very unlucky admittedly, but it really is rare to hear of that nut/shaft failure on here these days. As long as that new 12mm nut is used then it seems to have done the trick.