Collective Wisdom - Teen Time

OK - here's the next collective wisdom request - Teens and time.  I have two boys, one teen, one almost teen and it seems that we are always arguing about time - particularly "family time"  I am thrilled to have boys who enjoy friends and have, shall we say, an active social life.  In some ways, I'm envious because I've always been an introvert and socializing can be tough for me sometimes.


Having said that - I think there is a balance somewhere between social activities and family time.  We get into it sometimes when we are trying to go to the cabin or other travel.  We often have them bring friends which we all enjoy, but sometimes I would just like us to go as a family.  


The struggle seems to be that perceived fairness - when do I hold my ground and just say no to the activities and when do I give in and change the plans.  I know it may sound easy to just insist, but, perhaps because I'm an introvert at heart and may be too prone to some quiet time, I think I should be more flexible occasionally.


So - how do (or did) you manage it with your teenagers?  Have you found a balance or system that seems to work well?  


Again, all opinions and advice are welcome here - feel free to share.