I think you aren’t really hitting on the main point… I think since forever people (particularly kids growing up) have been very insecure about who they are as they are still trying to figure themselves out and really understand where they fit into the world. 

This ‘trend’ isn’t really a trend as it has been around as long as the internet itself and there have previously been online communities built around this entire ‘rate a picture’ or ‘hot or not’ type of system. People ultimately turn to the internet for validation and opinions because such people believe that those closest to them are not being honest as to not potentially harm their relationship, thus they turn to complete strangers in the belief that compliments or criticisms which are given, are done so sincerely as the strangers have nothing to really ‘gain’ or ‘lose’. 
I think the underlying problem here is caused by high pressure from society, parents, and the media telling people that they can all be something great, a real beauty, something that will take other people’s breath away, and everyone wants to be that amazing thing in life, but that’s why growing up is so painful for many people because it requires a level of self-actualization and accepting that a lot of the promises and stories you were told by adults were just hopeful lies. 
Once we reach adulthood (at whatever age that may be at, as people mature at different speeds), we have a generally good idea of who we are and we have grown to accept who we are as we realize that in actuality, many of the things valued by society such as beauty are actually highly overrated and highly temporary attributes which will cause the person with said attribute to be used then thrown around once their beauty has exhausted so that the next person in line can take their spot while they rot in the gutter, at that point, people realize what the truly important things in life are things such as life itself.