Why do websites such as Facebook and YouTube constantly update their layouts when nothing is wrong with them?

Question:

Facebook has gone through almost one layout change every year for the past 4 or 5 and YouTube is constantly changing.

Why do they keep changing Where a website like Google is constant?

Answer:

They make changes to make money. Product teams inside of these companies are responsible for KPIs, or Key Performance Indicators. They often have very specific goals that feed into broader business goals.

Facebook is extremely data driven. They might have a metric that tells them that people who post photos at least twice a month spend 10% more time on the sight than other users. So they will devise some UI changes to encourage the posting of pictures and then measure the impact against their large user base.

These companies aren't just bored or trying to 'stay fresh'...they have very specific business goals and have the data to back up the changes that they make.