Facebook is a great tool to keep up with the lives of your friends you don't see too often. Of course i have dozens of friends on facebook who I keep on my friend list just so I can easily contact them if needed. It's easy to hide them from my news feed if I'm not too interested of what they're doing. Also I can choose not share my stuff with them also.
People using facebook should sometimes really think what they allow other users to see. I sometimes see wallposts from person to person, which in my opinion contain information that shouldn't be shared with others. I think that most of users don't even realize how much stuff they are exposing to other people.
Facebook provides good playground for stalkers, provided that user has left his/her account to be seen by anyone. I've already heard some of my female friends complaining about people stalking them on facebook. This of course can be avoided simply just restricting your profile options.
Social medias and especially facebook provide a great opportunity for advertisers. Advertising can be focused more and more individually thanks to your profile information. Also groups offer a great place to focus your ads to people who are more likely to be interested in the product or service advertiser is offering.
Bands and movie companies are nowadays providing a lot of information in advance of their albums and movies through their own profiles and groups. I think this sort of marketing is more effective because users can choose to see these posts. It makes advertising more individual experience, especially when the user actually wants to see the ads.
In general social medias give great opportunities to advertisers, even smaller ones. With a little effort you can for example create a group for whatever you're doing and the people who are interested in your thing may contact you.
Facebook has a lot of potential in marketing and bringing people together. It will be interesting to see what kind of gimmicks we'll see in the future in facebook advertising and in general in facebook. I just hope while waiting for that, people remember that there's also life outside the door, not just in your computer.