Facebook is the biggest social site with over 250 million users. It took Google ten years to reach 50 million users – something Facebook achieved in just 19 months.

Facebook is the world’s online scrapbook and meeting place in one. Friends, customers, stars, fans, and anyone who crashes the party use Facebook to stay in touch, swap photos, pay homage and talk to each other.

At a recent meeting, a client told me ‘I don’t use email anymore, I only use Facebook’. Everyone she communicates with (including for work) is on Facebook, so why leave?

Facebook is viral by default. For example, if I post and comment on a photo, my ‘friends’ will all see it on their Facebook pages. Then if any of them comment on my photo, then their comment, my photo and my comment will all be seen by their friends. With one post, I can start a conversation with my friends and all my friends’ friends. That’s powerful.

If you have a large network on Facebook and talk about something interesting enough or present a Facebook application (like a game or a quiz) that users like, then it can spread across the internet. This is what happened with the Liverpool St Station flash mob.