Yesterday I excitedly typed on Facebook to the effect of “I’ve just found this amazing travel blog by some foreigners living in South Korea”, then forgot to link to it. Soon after, a friend replied to the effect of “Well, aren’t you going to tell us which blog?”.
In some contexts this can be used as an informal and emphatic alternative to a/an: “I’ve just found an amazing travel blog” v “I’ve just found this amazing travel blog”. In others, it is definitely demonstrative; I am pointing to or showing or linking to ‘this one here’. Without spoken tone, it would have been impossible for my friend to understand which meaning I meant, but either way, he would have expected a link, or at least more information.
But sometimes the demonstrative use isn’t possible. If I say “I ate this amazing pizza for lunch”, I am certainly not pointing to or showing you the pizza. “I ate this amazing pizza for lunch” can only be informal emphatic; compare “I ate an amazing pizza for lunch”.
The blog was this one, which, having explored further, I’m not quite so excited about, but it’s still definitely in the top 5.