I find advertisement very interesting subject.. After Facebook :) nowadays at most of the places I read 'no advertisement' Actually what is advertisement? I think we all are doing it. I visit places and take lots of photos, and sharing them aren't kind of advertisement? Even this is not my purpose.. If one travel agency shares same photos that will considered as an ad for everyone of us, because they earn money from people who travel..
I think travel agencies are doing an open advertisement, people like me are doing covered advertisement.. (me and people like me don't have any related purpose of travel agencies have). I think everything we do on this internet can be considered as some kind of advertisement.