Hey Poornima,
Social networking sites may be used as a huge database; however, they have their own pros and cons. For instance, you may observe a person's activities and their network before approaching them. Thus, you will have a behavioral sketch of the person. However, since most users don't portray their real selves, it can be misleading as well. Therefore, networking sites may be assumed as data pools and nothing more, and hence, it's not advisable to use these sites for critical recruitments.
Cheers,
Devashish.