They can be a waste of time. They can be used for stupid things, like online games, silly quizzes, etc.

On the other hand, it can be a very useful tool for keeping up with friends and family. A lot of people say, "If I want to keep up with somebody, I'll just call them." I'm going to say that's a load of bull crap, because life gets way too busy to keep up with everybody you'd like to keep moderately in touch with. Sure, siblings and best friends you'll probably call, but even with them a quick perusal of Facebook can keep you updated on more mundane things.

A few examples of how I've used Facebook.

1) After not hearing from anybody I taught from my mission for 27 years, I was contacted by two of my converts within 1 month of each other. Both are very active, and really wanted to track down "their" missionary. It has been a wonderful experience seeing pictures of THEIR children going on missions. I've shed more than a few tears of joy seeing how the gospel has blessed their lives, and Facebook is what has facilitated this. Sure, it's possible other ways, but Facebook was a very useful tool here.

2) Following on #1, and listening to the counsel of Elder Anderson in GC a few years ago, I decided to try tracking down a few of my other converts through FB to do a little bit of missionary work. Some are active, some are not. Even with the ones who are not, it has been a very joyous reconnection. I have become friends with several of the children of one of my converts who never knew their mother had once been a member. Just last Friday I had a 30 minute chat (in Spanish) with the 18-year-old son of that convert. He's not interested in the church right now, but he knows WHY I was in his country 12 years before he was born, he likes chatting with me, and he was excited to hear my son his same age is going on a mission.

3) I have a very large extended family that mostly live in AZ, whereas I live in UT. I don't get to see them that often, and most of my 46 nieces and nephews I just wouldn't find the time to call. Through Facebook, I get to learn good things about their lives--graduating from college, having children, etc. I'm not the completely absent, unknown uncle just because of distance: I get to know them and be known by them even if it is just online. I have a niece struggling with her testimony--I wouldn't have had any idea without Facebook, but since I know, I've been able to offer support and advice that she has appreciated. I have another nephew who has been to prison, and has separated himself from the family--won't talk to most of the rest, but he communicates with me through Facebook. He has thanked me many times for being "the one family member who will listen", even though in reality HE is the one that broke off contact. Facebook gives him the distance he needs, and yet allows me to keep the contact. He has even told me he loves me, and if you knew him that would shock you.

Like I said, FB accounts can be a waste of time, but they can also be a very useful tool. It just depends on the user more than the tool itself.