Facebook is initiating an advertising exchange that will let advertisers reach their target audience via the Facebook site.

Facebook Exchange will be a new revenue boosting measure for Facebook, one that is sure to parlay investor fears, to some extent; especially after the company’s turbulent Facebook IPO late last month.

Here’s how Facebook Exchange works: suppose you’re browsing a travel site to check flights to Heathrow but you don’t end up purchasing one.  The travel site can drop a cookie into your browser, and now, when you return to Facebook, you will be “retargeted” to see display ads for discounted tickets to London.