I'm not a huge fan of social networks or tagging but I wondered for some time why social networks like Instagram and Twitter don't have "Global" tags and "Personal (Local)" tags. Let's take a blog for example; tagging on open web blog is by default "Personal" or "Local" because your blog is not in a some kind of network with other blogs so you are basically tagging to better organize your posts in a form of list or collection. But social networks are completely another story; your account is your social profile that is a part of a wider network or social graph but by default your tags are "Global" which is probably meant for content discovery but somebody else can hijack your tag and there is no way for you to let's say: Share a pic of a cat and tag it "Cat" as your personal tag which would link to your other cat pics which are tagged "Cat" and you could also tag the pic with "Global" "Cat" tag which would link to global discovery feed of cats for the whole social network.