There are certainly cases of naysayers besmirching companies and entrepreneurs. There are also cheerleaders who get sucked into hype and think every new thing is going to change the world forever.
The truth of course is somewhere in between. You can't judge people or what they say on whether it's positive or negative but on the merit of what's said.
I for one have been critical of Groupon [1], even before the S1 filing. Large funding rounds to pay out early investors prior to an expected IPO are always a red flag for me but it goes way beyond that: dodgy accounting practices (ie treating customer acquisition as an extraordinary cost), account risk, the payment model that encourages last-ditch efforts by failing businesses to stay alive, the fact that Groupon's model in reality suits very few businesses, customers aren't particularly loyal or valuable and so on.
As for Twitter, my personal opinion is that Twitter is one of those phenomona that is magnified in the Valley bubble. Outside of the Valley, it seems it's used to follow celebrities and most people don't know about it. I believe their own user figures are exaggerated (Twitter publishes figures on registered accounts not N-day usage like Facebook and others do). Also the 140-character limit and the huge reliance on third-party clients present huge barriers to monetization.
I can't say I've ever said anything negative about Zynga. They produce games that operate (very well) on human compulsion. All they really need to do is keep introducing new games and refreshing the old. The one huge danger for Zynga has is their almost total reliance on Facebook.
As for Google+ (disclaimer: I work for Google), I like what we're doing here but success is by no means guaranteed. The biggest problem for Google+ (IMHO) is that people are tired of using social networking not that people are tired of using Facebook. I also believe that Google does a much better job of respecting user data and privacy and it would be good for the Internet as a whole if Facebook where not to be a monopoly on identity and social networking. None of that guarantees success. Or failure for that matter.
I also think the value of social search is completely over-exaggerated. The biggest innovation in the early days of the Internet was that suddenly you could find information from everyone in the (connected) world, not just your circle of friends and whatever print publications you had access to. The idea that we want to go back (even in part) just seems backward to me, especially given that most people consume information rather than produce recommendations.
What makes Google so successful in advertising (compared to any other company on the Internet) is intent. The act of searching for something means you want to find out more about it, find where to buy it maybe, etc. Let's face it: advertising in almost all other sites is an intrusion and a distraction that doesn't match out intent (of, say, sharing photos, posting updates or whatever).
So count me as being negative on Groupon. I think of all big IPOs for some years, it is the dodgiest and it strikes me that they're trying to cash out before the bubble bursts. To attribute my reaction as some flaw in human psychology that simply wants to drag others down however is rather dismissive of the arguments I make and those made by many far more erudite than I.
[1]: http://news.ycombinator.com/item?id=2870186