“This” is what’s present, what’s before us, what’s near and dear to us.
“This” is a form of direction, a continuation of location, just like “at.” In asynchronous online discussions, "this" functions as a quick means of support for the words of another. "This" is a one-word affirmation of someone else's expressed opinion (see Fark.com).
Like the previous three words addressed, “this” can be and often is defined by a moment. There’s an appropriateness to “this,” and it happens by pose and by choice.
“This” is who we are. “This” is what we do.
“This” is a role, established by self and others.
“This” is a performance. “This” is attention and recognition from others.