THE INTERNET IS UNHEALTHY
Let us state the not so obvious...
PHYSICAL
Why is this such a short section? Cus this is the obvious bit! Sitting all day on a computer is obviously a cause for concern. Not exercising is one of them. Leaning towards your screen is bad for your posture and may cause back pains. The LCD screen of your computer will sure cause eye damage. Not to mention the radioactive electro-magnetic field that apparently reduces the effectiveness of sperm. Do you want kids guys? WELL BETTER NOT PUT YOUR LAPTOP ON YOUR CROTCH.
Addiction
Have you ever went through a day without checking Facebook or Twitter or any other social media site? No? Well then you sir/madam are addicted! This can affect your lifestyle and may get in the way of school-work and even social interaction. It is the main cause of procrastination now, and is perhaps the most effective in reducing the amount of physical interaction with actual people. Internet addiction also causes sleep deprivation, which affects the effectiveness of the brain and response-time.
Cyber-bullying
Usually residing in social media sites, this is a form of bullying that perhaps you cannot ever escape (unless you live in a hole like Bilbo). Things such as rumours, gossiping, and posing as someone else online are some of the most common forms of cyber-bullying (not to mention the classic frape).
WIKIPEDIA
DO NOT TRUST WIKIPEDIA!!! Ok, maybe wikipedia is alright, but remember that anyone can edit the content. That's right, anyone can post a false fact on the internet and claim it is "accurate". Which one should you choose? Which one is reliable? Which is the most accurate? Be careful where you search for information. Check the sources, check the background of the source, compare it to other sources. Or just stick to the old fashion book.
BEHAVIOUR
Without the actual face-to-face interaction that you usually get when talking to a person, you tend to get a little less sensitive in real life. Aggressiveness in language and expression is one result of having lack of physical interaction with a human being, and being shy and anti-social is another. This affects your lifestyle drastically (and possibly the amount of actual friends you have; how many of your Facebook friends do you really know well?).
"INAPPROPRIATE" BEHAVIOUR...
Pointing to most of the guys, pornography is probably leading in the online marketing business. There are a lot of things that google can accidentally show you, and quite easily accessed too. Things like sexting (sending inappropriate photos of yourself around) is also a huge no-no. This will most definitely affect your future life, especially when your parents see your nude online.
PRIVACY
Once you open up your browser and type in a website, everything is immediately recorded. Whether it's your browser recording your history, or the NSA tracking your terrorist activity, you're never anonymous. Nothing you do is private, and everything can be accessed. Be careful where you go. Do you really want your friends to find out you were searching for twerking tutorials on youtube? I thought so.