Ok so this is a big problem with the emergence of social networking most people aren't meaning what they say over the internet. Most of the time people are joking but of course you cant tell over the internet.
Here is what you can do to prevent this:
1) Before you post something read it over
2) Think would you want someone to say that to you?
3) Keep in mind is that person sensitive
4) Re-read it one last time make sure it is appropriate (cause colleges and jobs look at your social networking)
If you accidently post something you think is hurtful
1) Delete the post
3) Tell them you didn't mean it
Its not much of a problem here on TC but I have caught myself doing it. Thanks guys Peace