There are whole field of scientific study based almost entirely on self-reports. As such, there has been much research done on its veracity, which has shown that it is largely valid and reliable when reporting on simple things (like friendship behavior) so long as it's anonymous, does not require introspection(1), and there is no fear of reprisal(2).
The 2007 study meets all those requirements, so I see no scientific reason to question its results.
Also, I can't think of any practical methodology for studying friendship behaviorally - but would love to be proven wrong.
1. Baumeister, R. F., Vohs, K. D., & Funder, D. C. (2007). Psychology as the science of self-reports and finger movements: Whatever happened to actual behavior?. Perspectives on Psychological Science, 2(4), 396-403. Baumeister, R. F., Vohs, K. D., & Funder, D. C. (2007). http://users.business.uconn.edu/jgoodman/mgmt%206201%20assig...
2. Brener ND, Billy JOG, Grady WR. Assessment of factors affecting the validity of self-reported health-risk behavior among adolescents: evidence from the scientific literature. Journal of Adolescent Health 2003;33:436-457. Summary at http://www.minnetonka.k12.mn.us/TonkaCares/RwR/Documents/Val...
The 2007 study meets all those requirements, so I see no scientific reason to question its results.
Also, I can't think of any practical methodology for studying friendship behaviorally - but would love to be proven wrong.
1. Baumeister, R. F., Vohs, K. D., & Funder, D. C. (2007). Psychology as the science of self-reports and finger movements: Whatever happened to actual behavior?. Perspectives on Psychological Science, 2(4), 396-403. Baumeister, R. F., Vohs, K. D., & Funder, D. C. (2007). http://users.business.uconn.edu/jgoodman/mgmt%206201%20assig...
2. Brener ND, Billy JOG, Grady WR. Assessment of factors affecting the validity of self-reported health-risk behavior among adolescents: evidence from the scientific literature. Journal of Adolescent Health 2003;33:436-457. Summary at http://www.minnetonka.k12.mn.us/TonkaCares/RwR/Documents/Val...