all the facebook groups i have seen have been very moving, usually set up by close family members or best friends... i think it can help people cope with the grief..
for example i saw one started by the life long best friend of a young man who had died and it was obvious he was struggling immensely (and the family of the young boy).. and this was a way to help him remember and cope..
i've also heard of parents who have lost a child setting up pages to help show memorial
it's not new, people have been setting up memorial websites since the internet because popular.. facebook is just an easier and acessable way to do that.
i think different people do things differently. they don't cause harm so if it helps some people then there is no problem. if you don't like it remove it from your feed and don't visit it.
i know how you feel "they didn't even mean it" about that guy who posted on the website... when my very close family member died, only one or two of her supposed "best friends" came to the funeral. i was very angry with them for that, espeically as they obviously knew about it. then when they say "i'm sorry about **** dying" or something similar it just seems insulting..
but in the case of the guy you saw it doesn't sound like they were very close, i don't know why he bothered posting but it is the same as telling an aquaintance that you are sorry someone they loved died... you are sorry that they died but you don't really feel emotional about it because you didn't know them. just paying your respects.