I guess the interesting part to me is that, with the advent of news aggregators and social networks, the internet kind of removes the filter. A blatant lie can take off like wildfire and burn a lot of people before anyone can mobilize to put it out. Maybe I give journalists too much credit, but before the advent of the internet as we know it today, these organizations (whether big oil or Green Peace), had to filter the information through some sort of intermediary first. Now they have direct access to millions of people.
Yeah it's definitely a double-edged sword. Like the person whose house was posted to Craigslist with a "come and take what you want", which people did....and only too late did the owner find out -- and of course, he made no such offers. But the mob was quick to act and didn't ask many questions.
On the brighter side, there's quite a few exposés that we now witness that would have never seen the light of day otherwise (or would have been relegated to B-theaters, flea markets, and mail-order catalogs).
No. Those are pretty orthogonal notions (if you're talking about the idea that you excerpt existing pages that's not really what semantic web means nowadays), and the semantic web remains a terrible idea.
What we need is a more efficient legal process for online statements that cross the line into defamation, and better collaborative filtering (the whole point of sites like reddit is that the truth should out).
The dangers of the mob are very real though, as many people's lives have been ruined (or nearly ruined) through some misinformation that goes viral.