Many are aware of Amazon’s robust retail and web-hosting businesses, but less are familiar with its advertising business, which is rapidly becoming a significant player in the space. Amazon brought in $450 million in advertising in revenue last year, according to eMarketer estimates published Tuesday. The company managed to increase its advertising business by nearly two-thirds between 2011 and 2012, and is expected to grow it by another half this year — to $660 million. This makes Amazon’s ad business even bigger than Twitter’s, which eMarketer previously estimated would generate $580 million this year. By 2015, Amazon is predicted to bring in $1.1 billion in advertising revenue, about one-fifth of Facebook’s earnings last year. Advertising is still rather small part of Amazon’s total revenue picture however: The company reported $61.1 billion in net income last year.