If you are just wondering why the whole Facebook is filled with rainbows, it’s because the U.S Supreme Court ruled that same-sex marriage is now legal across the United States. The rainbow flag is used worldwide as a symbol of lesbian, gay, bisexual, and transgender (LGBT) pride and LGBT social movements.
If you go to this link, www.facebook.com/celebratepride, the tool will show a preview of your profile pic with a rainbow filter. You then have the option set this as your of profile picture.