College admissions officers, prospective employers and even attorneys check people out on social networking sites.
1. It's a bad idea to post photos which are racy/sexually explicit or depict unwise behavior, i.e. binge drinking.
2. Don't write disparagingly about a college you might want to attend OR a company you might want to work for.
3. Do not post negative comments about your job(s). Prospective employers may see these posts.
The following articles discuss some of the potential hazards of social networking:
5 Dangers of Facebook
Teacher Loses Job Because of Facebook