Intimate photos or videos:Anything that is once posted online can never be deleted completely.So make sure that you donot post something that you may not want your colleagues, boss, siblings or parents to see.
Your phone number: And for that matter even your address. Even if you think that you can trust you e-friend, donot be naive. Similarly donot give out you address for some lucky draw. This very lucky draw may prove unlucky for you.
Donot post any abuses: If you have some complaints against you boss or teachers donot write messages against them over the net. For starters, it can be easily traced back to you. Would not you rather talk about your complaints face-to-face with your boss than miss out on that promotion you were vying for.
Bank details: Its one thing to pay your bills online over secure websites, but never ever can you give out your account number or your ATM pincodes. Donot take such risk no matter what the emergency is. If someone asks you for such details be suspicious of them.
Minute-by-minute details: Its one thing to tell people how you feel or crack a joke a two over the net, but donot give out details like going out for groceries or heading to the local bar.