Why are people weary of giving e-mail and their facebook link?

Nowadays, its not just girls I talk to online weary about giving out their phone number and address to a stranger but Why is it when I ask if she has facebook and for her facebook link, she is even afraid to give out her facebook?! Including her actual e-mail? If it was AIM and just talking by the dating site e-mail, she's ok with that but she is afraid to give me her phone number, her name on facebook to add her, and shes even afraid to give her e-mail? how is e-mail and facebook so personal and any more personal than a number?