I give my students my cell # (not my home, I barely use that one, and have a housemate, so that wouldn't work). I used to give them my personal e-mail address (to send me their assignments, or in case I needed to do a mass mailing), but with spam and facebook accessibility, I stopped that. They now contact me on my work email (and i am better with checking it on my days off).
I often text my students, and they text me. I have their names in my phone, so I know it's them when they come up, and I can also send them a group text if needed. It works better if they are calling in for clinical, or running late, so I am not picking up my cell phone on the unit.
Once they graduate, I see no issue with keeping in touch with them, in any way. They are no longer my students, I am not longer their teacher, they are now fellow nurses. I have a number of them on my facebook page, and I really do enjoy seeing their nursing-related gripes