"Ian: the fact that you get +some+ value out of me, but too much noise to deal with, tells me the tools are broken. "
And it tells us you produce noise.
"If you turn noisy I just put you in my noisy folder. That usually solves the problem"
Noise happens *at all scales* and for social media is *relative* to the state of consumer not just the producer. That's why neither your or Ian's position ("it's the tools" v "it's the social") is the right one.
"URIs are globally scoped, which means they need to mean the same thing in any context."
isn't true, for RDF. URIs don't have meaning they have denotations; denotations are assigned ("distributed") and that can be done in a local scope. In theory, when you merge data, you determine that the same URI has different referents via logical inconsistencies; in practice you have domain experts and data modellers look analyse the data (just like you do with relational database integrations).
For me, you left out an most important thing, which is lots of URIs in the same place are hard to read. QNames win the readability argument.