1. Waste of time
Let's face it. Facebook use is almost always going to be for fun, out of boredom, to communicate with friends/family and share posts and pictures. Usually there is something else you could be doing with your spare time that is more productive. Whether it be the dishes, the laundry, your homework, your taxes, a job application, an appointment, and so on. Very rarely if ever the average person will use facebook for some kind of productive purpose that makes progress in their life, other than maybe reconnecting with an old friend, family member, or lover. (Which may not even be a good idea afterall)
2. Privacy and discretion
I can't tell you the hundreds and thousands of things I have learned about people that I have met, or have never met, just from reading their facebooks. Some of these things are very personal, embarrasing, or just downright unintelligent or self-damning. People seem to get the idea that if you are not friended to them on facebook, you are not reading their facebook. THIS IS NOT THE CASE!!!
In fact the only reason I even use facebook is to read other peoples' profiles and find out information that I want to know. It is a very useful investigation tool for sure. If I do happen to mention something to someone that they never told me, they often wonder how I knew about it. *smirk* People don't realize that it's not just their facebook friends and family that read their shit. There's people you DON'T want to know certain private things about you. Everyone has skeletons in their closets, and it seems facebook has been a pretty useful tool in unveiling them.
Most facebook profiles are set as public. However, even when people set their facebooks private, it's actually not private. You can still browse through tons of posts and pictures through mutual friends. So if you have one friend at all on your profile, IT AIN'T PRIVATE!!!
3. False advertising yourself
People are often a lot more bold on the internet. This can range from what they say on their posts, to private messages, to the pictures they put up and who they friend. If you read somebody's facebook profile before you get to know them much, it can be easy to get a preconceived notion about who is this person really is. This could be good, or bad, but either way there's a great chance of it being inaccurate. As mentioned in the previous post, it's another form of privacy invasion. Some people might friend you just so they can read your posts and use them against you. This can also come into play when applying for a job. There's plenty of people who have not been hired over things they posted on facebook years ago, or fired for current posts as a matter of fact.
4. Lack of Communication
With all of this technology, the main way of communication is now through an electronic device. This will just decrease the amount of family gatherings, barbecues with friends, and maybe even dates or love letters from your significant other. It is far more personal to be able to talk to someone on the phone or meet them in person.