A 600-person sales office in the heart of New York’s advertising district — that’s VentureBeat‘s expectation of a two-floor lease Facebook signed at 355 Madison Avenue, assuming that facts were correctly spilled to the blog off the record.

An unnamed source told Owen Thomas that Facebook could expand to as much as 150,000 square feet of the mid-town Manhattan building.

Apparently, the company already has a 5,700 square-foot office in the neighborhood, and we don’t know how many people work in that space. However, the company lists 15 job vacancies in New York. Most of these positions are sales or operations.

It looks like Facebook wants more people within walking distance of New York’s advertising agencies, especially the ones that plan the largest media campaigns.

This presence would certainly give the social network more of an edge over Google, which apparently hasn’t always hit it off with the bigger players in branding and media buys.

Readers, what do you think about the real estate play in Manhattan?