That there are significant differences between personal social networks, like Facebook, and professional social networks, like LinkedIn, seems obvious. But what are those differences exactly, and how do they manifest themselves in terms of actual usage?

To answer that question, LinkedIn turned to research firm TNS to conduct a survey of social network users and determine why they use the personal and professional social networks.

The ultimate finding of the study: there’s a huge difference between the two types of networks in “purpose and mindset.” On services like Facebook, users are far more likely to “spend time,” whereas use of networks like LinkedIn is considered an investment of time. In numeric terms, LinkedIn and TNS found that “people are more than 3 times as likely to use personal networks for entertainment rather than professional networks” and “3 times as likely to use professional networks to keep up to date with their career over personal networks.”

This may not be all that surprising, but it apparently does have an implication that could be significant to the operators of social networks: according to the 6,000 individuals polled, updates provided by brands are the second most desired form of content on professional social networks; they barely make the top ten on personal social networks. All told, individuals on sites like LinkedIn expect to hear from brands 26% more than on sites like Facebook, suggesting that the stock market’s treatment of LinkedIn versus Facebook may be quite rational.

Social’s impact on job search

The differences between professional and personal social networks extend to job hunting. According to a study released today by Millennial Branding and Beyond.com, Baby Boomers, somewhat surprisingly, are more likely to use social networks to search for a new job than their Gen X and Gen Y counterparts. Also, interestingly, members of Gen Y and Gen X are far more likely to use Google+ for the exercise than their Baby Boomer counterparts, who are far more likely to use LinkedIn.