For example, as Barnes (2006) points out, users of social networking sites voluntarily surrender private demographic information that can be capitalized on freely by marketers. Aggregate marketing data can be bought, sold, and traded to provide the most robust data set in the history of humanity. Because of the power of social networking to create a vast marketing database, it becomes more and more important to study the particular effects of social networking on marketing results. It will be argued that while traditional word of mouth remains meaningful, that social networking may replace traditional word of mouth because of perceived credibility. As social networking becomes an increasingly viable forum of social interaction, consumers start to trust virtual friend. Consumers will view virtual friend opinions as having the same credibility as the same reaction occurring in a face-to-face situation. In fact, with social networking the consumer is empowered by the Internet and able to conduct...

This could mean that social networking will surpass traditional word of mouth as a means of disseminating information about products and services.
The best way to narrow a topic as potentially rich and rewarding as this would be to focus on a specific target. For example, the research could compare word of mouth and social networking inputs to an output of a fictitious online company created for the purposes of this research. A fictitious company has advantages over a real one in that it would eliminate extraneous variables.

References

Barnes, S.B. (2006). A privacy paradox: Social networking in the United States. First Monday, Volume 11, Number 9 -- 4 September 2006

Trusov, M., Bucklin, R.E. & Pauwels, K. (2009). Effects of Word-of-Mouth vs. Traditional Marketing: Findings from an Internet Social Networking Site. Journal of Marketing 73(5): 90-102.