An expires header is meant to say how often the browser should check for new images instead of just using what it has in the cache for the specified image. Since that's server dependent, there's nothing you can do about it. Those headers would have to be added by Facebook. They intentionally leave the expires header at a very short period so that the browser, for those files, checks frequently for updates. This is intended because it makes sure that their Javascript API running on the user's browser, or the avatars of the users that are displayed, are always up to date.
So, in short, there's nothing that can be done about those headers. However, I don't think those files should be the cause of any slowdown on your page.
Hope this helps!
Add Expires headers
