Where to start? - I accept that millions of people use Facebook (my wife and daughter amongst them, so I do know how it works), but I'm not one of them, and I think that the potential security and "spam" considerations outweigh any potential "benefits" - and I do differ that "it's a great medium for contacting members, advertising events" - because it (by definition) only reaches those who fall for the "sign up before you can read it" blackmail - I'm not alone in resenting this form of coercion, so if you really want to reach everyone, you don't use "Berkbook".... Sometime the owners will probably sell your details to the highest bidder so they can spam you...
Facebook isn't the only one who uses these tactics - Yahoo (and similar companies) are a total pain, and have a history of liasing with spammers - I'm forever getting requests from people to join them on "Linkedin" - you go to the site to tell them to sod off, and you have to "join" before you can do so..........
Flickr is part of the Yahoo empire, and you can scarcely move on the 'net before their admonishment to "join" their pernicious organisation surfaces.
Whilst I'm having a grumble about big internet companies, even Google should be treated with care - they're "curate's egg" - their search engine is by far the best, but they do hold an inordinate amount of information on all of us, and I don't believe that they display the necessary straightforwardness desired (their arrogant "streetcam" mapping shows they have nil regard for privacy or potential criminal usage) - I don't like any organisation holding the amount of private information that they do, it has enormous potential for doing harm - not least handing those details over to a less than benevolent future (or present!) government...
If anyone remembers Hill Street Blues - "be careful out there"