Facebook – Posting YouTube video to Facebook

Till the end. I don't remember the source, but I read it somewhere in YouTube itself.

Update: According to this page in YouTube support, it's not that simple.

Prior to 200-300 views, view count on the watch page acts as a real time hit counter. Each visit to that page, regardless of who is visiting (even the video owner) increases the view count by 1 (but see the secondary system below).

After 300 views, the whole system changes; the secondary view counters kick in. YouTube has never said exactly what they're counting (or not) but it's known that they throw out the original hit counter and re-evaluate all the page views and only allows hits which YouTube calls a "view".

This is just speculation, but I would expect that they'll be counting all page views from the same IP address with a certain period of time as one view, obviously it's not as simple as this as a lot of people share IP addresses, there will probably be cookies involved as well. This will stop people artificially inflating their view counts by repeatedly refreshing the page from a single computer.

I don't know how long the time period would be - it may in fact be effectively forever so if you went back and viewed the page again in a year's time it still wouldn't count it as an extra view.

It's important that Google get this number to reflect (as far as possible) the actual number of people who've actually seen the page as they are selling advertising based on the page views. The more page views the higher rates they can charge. Anyone running a site that has advertising has the same problem to solve and a lot of time and effort goes into getting these numbers as accurate as possible.

They won't publicise the exact algorithms they use as this will allow people to game the system and artificially inflate the viewing figures.

I would also suspect that, like on Stack Exchange, the viewing figure as displayed is cached. So when you view the page for the first time even though it registered the new view the figure on the page won't update for a few minutes (or even hours).