Does Facebook activity predict job performance? I've seen a little bit of discussion in the media about  Facebook being used in selection and recruitment settings. 
Assuming an employer can gain access to a person's Facebook profile, have any scientific studies been conducted to assess the following questions:


Does Facebook activity predict job performance?
If so, what does Facebook activity measure as seen by correlates with existing scales (e.g., personality, intelligence, integrity, etc.)?
Summary of Kluemper et al 2012

I had a read through the article by Kluemper et al (2012) mentioned in the answer by John Pick. The following summarises some key points.

After discussing the broader context of using social networks to measure personality, Kluemper et al (2012) cited the findings of a couple of existing studies:


  Karl, Peluchette, and Schlaegel (2010) found that social network users high in
  conscientiousness were less likely to post problematic content
  (e.g., substance abuse, sexual content), while Amichai-Hamburger and
  Vinitzky (2010) found a relationship between self-rated extraversion
  and number of Facebook friends.


The study by Kluemper et al (2012) involved two studies. Each study involved getting ratings on around 250 participants including self-report Big 5 personality and then getting three raters to read through the participant's Facebook profile and then fill out a measure of Big 5 personality. Self-other correlations for the same personality factors ranged from .16 to .44. This supports the idea that Facebook profiles can be used to get at least an approximate measure of personality.  It should also be mentioned that these self-other correlations are a fair bit lower than those obtained by Costa and McCrae (1998) when using the NEO, and a single knowledgeable other-rater (e.g., in the r=.5 to .6 range). 

Meta-analyses have generally found a positive but weak correlation between Big 5 personality and job performance (e.g., Barrick &amp; Mount, 1991). In combination with the results from Kluemper et al, this may suggest that personality measures extracted from Facebook profiles might predict job performance. However, given the relatively low self-other correlations, this still requires further empirical testing.

Furthermore, extracting Big 5 personality using three raters reflects only one rating strategy. Thus, there are many other useful measures and measurement strategies that could be explored.

References


Amichai-Hamburger, Y. &amp; Vinitzky, G. (2010). Social network use and
personality. Computers in Human Behavior, 26, 1289–1295.
Barrick, M. R., &amp; Mount, M. K. (1991). The Big Five personality dimensions
and job performance: A meta-analysis. Personnel Psychology, 44, 1–
26.
Costa, P. T. &amp; McCrae (1988, PDF). Personality in adulthood: A six-year longitudinal study of self-reports and spouse ratings on the NEO Personality Inventory. It's not a meta analysis. N = 167 for spouse ratings.
Karl, K., Peluchette, J., &amp; Schlaegel, C. (2010). Who’s posting Facebook
faux pas? A cross-cultural examination of personality differences. Inter-
national Journal of Selection and Assessment, 18, 174–186.
Kluemper, D. H., Rosen, P. A. and Mossholder, K. W. (2012), Social Networking Websites, Personality Ratings, and the Organizational Context: More Than Meets the Eye?. Journal of Applied Social Psychology. doi: 10.1111/j.1559-1816.2011.00881.x URL