There are some trending posts on Facebook claiming that writing the word ‘BFF” into the comments of such Facebook posts can reveal if your account is at risk of being “hacked”.
The post claims that if the letters BFF seem in green, it means that your account is safe but if it doesn't, it means that your account is in danger of being hacked.
Though it's true that writing BFF into Facebook will, usually, lead to those letters turning green, this doesn't in any manner indicate whether your account is safe or in danger.
The letters BFF turn green when typed because they have been chosen by Facebook as a “special” comment that, upon being typed, turn out a short visual animation to appear on the screen. The letters informally mean “best friends forever”, and typing within the comment shows 2 hands “high fiving”.
Facebook has other special comments, including Congrats (or Congratulations) that turns orange and shows a short visual animation when typed as well.