Maybe a sales company needs a lot of salespeople... but what are these people selling? Advertising? I've been using Twitter more and more over the last year,

Maybe a sales company needs a lot of salespeople... but what are these people selling? Advertising? I've been using Twitter more and more over the last year, and I am hard-pressed to remember any ads that I saw. Who is advertising on Twitter? And is it really effective? And do they need 2,300 people to sell these advertisements?