Having worked in a marketing research firm, I have doubts about how accurate the surveys and polls they conduct are. Usually, they advertise something in a newspaper or media outlet such as "Craigslist". Problem here is that the potential surveyers are being lured with payment to participate in this process. Some might even make a side job out of this.
In a very recent article published in ABC news website, Standford researchers were quoted as being very skeptical about such surveys. A comparative analysis was done to see if there were any differences in results produced from non-probability survey versus a probability based survey. The later one was obviously found to be less accurate.
Even though the marketing resaerch firms' credibility is questionable, companies pay big amount of money for these types of services. The article reads, "online market research will reach $2.05 billion in the United States and $4.45 billion globally this year, and that data gathered online will account for nearly half of all survey research spending."


http://blogs.abcnews.com/thenumbers/2009/09/study-finds-trouble-for-internet-surveys.html