The Friendship Badge is a badge that is obtainable by having 20 friends. Users do not lose this badge if they have less than 20 friends after they obtain the badge. By far this has been earned by a lot of people and the easiest badge to get.
"This badge is given to players who have embraced the ROBLOX community and have made at least 20 friends. People who have this badge are good people to know and can probably help you out if you are having trouble."
In 2015, the badges got an update and the Friendship Badge was part of it.