Google has their Wallet platform which can be used to pay online (and offline) at supported retail outlets.
Facebook is working on a similar technology. The new feature would allow users to pay using their connected credit cards using their Facebook ID on supported web services.
The company already offer virtual goods and advertising services on their platform which means they have credit card information of thousands (if not millions) of users globally. They can use this to their advantage by offering an online payment service similar to PayPal.
Facebook has the user base that would make companies like Google and Apple jealous. It remains to be seen if they can take on the likes of PayPal in the online payments market. Google Wallet remains a relatively smaller player as it is popular in only a few countries globally.