50k visitors per month without any backlink

[frank]

I think he is using a paid marketing method for his blog like social media marketing and pay per click marketing because In SEO it is not possible to get this much traffic without any backlinks.

I agree PPC can be it.
As I had said earlier stumbleupon can be the answer. But then also the question comes that a sudden traffic spike may be possible but a regular traffic is not possible.