Features can sometimes only be visible in the aggregate. If facebook improves ranking of stories in a way that improves time on site and their user engagement metrics by 5% that will likely be invisible to nearly all users. Same for their ads. Plus they would be running many A/B tests, some making it onto the site, others only ever being seen by a small percentage of their users. Chat, integration of acquisitions, billing, APIs, 3rd party integrations, future products, etc.