Facebook, the social networking site, on Wednesday apologised for “mistakes” made in the roll-out of a controversial new advertising system and said it would offer users the ability to opt out of the service altogether.

In a message posted on Wednesday on Facebook’s website, Mark Zuckerberg, its 23-year-old founder, said it had “made a lot of mistakes” in building its new Beacon technology, which sends messages when a Facebook user makes purchases on outside websites.