Worldwide, the top 3 sites are Facebook, Twitter and LinkedIn. Facebook is far ahead with over 750 million visitors every month. Even if its shares didn't sell as well as expected, it is still hugely popular and still growing.

Twitter is getting more and more famous and may overtake Facebook one day, but at present it has only about a third of Facebook's traffic with a quarter of a million visitors. It's too early to say whether it will suffer from the bad publicity it's had recently, with people posting abusive or threatening tweets.

LinkedIn is surprisingly big considering it is more a business site than a social networking one. It has about 100 million visitors each month.

You can see a full list of the most popular sites here - Myspace and google plus are the next biggest. However, in the US the third most popular site is Pinterest, which doesn't appear on this list at all, so it does also depend on where you are counting.