I believe as long as they are reporting facts to go along with their opinons, there is no harm in investors being influenced by the blog. It can be compared to investment companies giving advice to their customers on what to invest in only this information is free. Investors have the right to choose who they trust to get information.
I do not believe blogs like Zero Hedge should strongly influence investor choices, but obviously it is investors who have to make that choice ultimately. It's their money so to speak and no one can force their hand any one direction. I think the stock market will eventually crash and there will be more to worry about rather than just a blog.
Blogs like Zero Hedge have always had a reputation for pushing investors either closer or further away from particular investments. This alone seems like nothing to cause any concern but recently these blogs have became much more powerful then they should. Blogs are meant as an outlet for a writer to voice their opinions, feeling, etc. and do so in an open forum. The problem occurs when you have such a large audience that your word actually becomes worth a dollar amount. You never know exactly who is whispering in these bloggers ears and when speaking in regards to the stock market, money tends to be made on investments when others lose on that very investment.
Blogs should not have such influence. This is because word of mouth actually just influences things that would otherwise be ignored. Blog posts are often rumors and hopes put into print. It is best for investors to look to company values and long-term growth to get the real story on a company's investment potential.