It was bound to happen. Facebook has just flipped the switch and turned on PayPal as an accepted funding source for Facebook advertisements. The announcement has been expected ever since Facebook and PayPal agreed to work together on allowing the use of PayPal to fund various Facebook expenditures.
The annoucement is important as it underlines a good working relationship between the largest social network in the world and perhaps the most important online payment processor. PayPal and Facebook have much to gain from working with one another, and perhaps more to lose by not doing so.
Facebook needs to grow the size of its advertising revenue, and a simple way to do so is to allow for money to be brought into it in a simpler fashion. PayPal wins as well, becoming even more so the de-facto method for paying for anything online. Also, as Facebook grows in size (as it continues to do at a shocking rate), PayPal will move millions through its servers to Facebook, collecting fees along the way.
PayPal is also a very trusted payment method. With it now being allowed on Facebook, it may open the door for people who were skittish about divulging their card information to Facebook but who trust PayPal enough to get into Facebook advertising. It is a good day for the Facebook and PayPal ecosystems, to say nothing of their users.