It's one place that accepts non-native engineers (I live in Japan), and it's very well-known/prestigious, so I could have a job at other places much more easily after working there. They also pay a lot more than most local employers. This is in addition to the technical challenge, etc. that other people already explained.
Personally I'm not hugely excited about it but it can be a good way to start a career.
That makes sense and that's what I can see; it's a good first stop. At least in the coming 10-15 years no one probably will fault you for having worked there as an engineer. Maybe longer.
I didn't know they accept non native engineers; that's another thing I forgot about; if a company really would like top talent, what's with the closed-border mentality? If Facebook doesn't have that, then kudos to them. If you want talent, it shouldn't matter where someone is from right?
Employees at Japanese companies typically don't speak English (this includes engineers), so they're hesitant to hire someone who may cause communication/cultural problems.
They also generally don't have the same attitude towards hiring and are definitely not hiring for top talent. People are hired because they fit in the company culture first, and because they have potential second. Young engineers are paid cheaply and are not expected to be skilled; they are trained on the job.
It's definitely possible to get hired as a foreigner in a Japanese company, but it's not easy and the conditions are not great, compared to Facebook et al.
Personally I'm not hugely excited about it but it can be a good way to start a career.