I’m grossed out by the kind of stuff tumblr does to its users over an extended period of time
Grammar and punctuation is completely lost on some people, to the point where their posts are unreadable and irritating
And tumblr fosters and feeds this incredible narcissism and need to impress people- people who don’t really matter in the grand scheme of things.
There’s no need to insert ‘that’ in the first paragraph.
I think that ‘completely’ should be removed, given the ‘to the point’ phrase in the other clause. On the other hand, I don’t think the comma between ‘some people’ and ‘to the point’ should be dropped.
'Stuff' is a poor word choice - the writer ought to be more specific, or even phrase it like “I'm disgusted by what Tumblr does ….”