as far as I know, "mereka" is used for the third person plural. And for the third person singular, we use "ia, dia, beliau (He, she for subject, and him, her for object)". For example, Maria dan Andi pergi ke pasar. Mereka ingin membeli buah-buahan. Maria and Andi are the third singular, also antecedent from the word "mereka" (third plural person)
Notification from facebook sometimes shown up ....juga mengomentari status mereka. I think this notification only shows/appears if we comment on one page in facebook, then the page also comment to their post. for example, I commented on page "the difference between american and british English, and then the page also commented on their status. exactly in my notification it will be "the difference.. juga mengomentari status mereka. Don't be bothered of this because sometimes facebook translation is not really accurate.