If you dont know how to delete your previous/old facebook comments then i will show you here , How you can delete your previous comments on facebook .Sometimes we forget where we have commented and for security purpose it is very necessary to check your previous comments .The question is that comment is yours or not for this reason you must know “Where to check your previous comments in Facebook”.
Follow these steps :
1- First go to menu of Facebook and Click on downside white arrow
2- Then look for “Actvity log” Option below Adsvertising on facebook
3- Click on Activity log button
4- Now you will see below page on your facebook Activity log page
5- Now here you can delete your comments or unlike your liked post
What is Facebook activity log
Facebook activity log keeps your record of comments and likes .In Activity log page you can delete your previous comments and remove likes from that page where you have done these tasks..so facebook activity log is very important feature of facebook.