Facebook Inc (NASDAQ:FB) and Twitter Inc (NYSE:TWTR) are both experimenting with on-site and in-app purchase options that would allow people to buy advertised products without having to navigate to another website, reports Julia Boorstin for CNBC. Twitter’s Buy Now button, powered by e-commerce site Fancy, debuted earlier this month and Facebook announced its Buy button in a July 17 post, but it’s currently limited to small and medium sized businesses in the US. Tracking purchases improves service for advertisers While you can expect both companies to take a cut of any sales that pass through their respective platforms, that doesn’t…