The teenage years are when your children typically begin spending much more time with their peers, rather than with the family. In fact, learning peer socialization is one of the key milestones of teen development. On a less intellectual level, navigating the daily changes of his/her social circle will simply become one of your teen's most engrossing activities. Rather than being too concerned that your teens are spending too much time with their peers, the larger issues are who the peers are that they're hanging out with or if they're spending too little time with peers. (Of course, if you never see your teen, know where he/she is, or who he/she is spending time with, you do want to address that. However, even that is more an issue of parenting oversight, rather than a teen spending too much time with friends.)
One way you can stay involved in your teen's socializing is to make your home a welcoming place for your teen and his/her friends. This way, you can keep an (unobtrusive) eye on how your teen's peer group acts. You can also take a nonjudgmental interest in your teen's friends. Don't interrogate them, but just show an interest.
In order to guide your teen toward the type of friends you'd like him/her to have, guide your teen toward those activities where such teens are likely to be. Not only will these activities connect him/her to like-minded teens, but the activity itself will let you know where your teen is and he/she is doing. At the opposite end of the spectrum, if your teen is not socializing at all you do want to figure out why. Is your teen overscheduled with school, activities and a job? If so, your teen might be in contact with peers, but not truly socializing. Is your teen excessively shy or even suffering from a social phobia? If your teen avoids social situations, talk to him/her about why and slowly encourage him/her to get out there. You don't want your teen to miss out on socializing entirely.