Facebook gives you the option to always use https, which obviously is a lot safer, but why do they give people the option? Why don't they just force everyone to use it? Is there ay reason why you wouldn't want https on? It looks like https doesn't impact performance too much.
Https is more processor intensive and slower as it is harder to cache etc because everything that is sent is encrypted so you can't share with others. Also encrypted data tends to take up more KB. Its a particular problem if you are encrypting images.
People who are still using slow connections may wish to avoid https use.