I use just about everything, because everybody I know isn't all on the same medium.
Text messaging is probably the most ubiquitous, because every throwaway cell phone comes with it these days, but AIM is making a bit of a comeback now in some of my circles (being one of the more prominent free IM clients for iPhones & iPod touches).
There's also Skype, which I use a lot for work, and then there's finally the occasional e-mail (<oldguyvoice>I remember e-mail! Ha! Back in my day, we had to...ahh, screw it, you remember the rest of that line</oldguyvoice>)
I used to not get the idea of texting on a cell phone, because I was always at a computer and it was easier to IM, and if not, then a phone call was cheaper. But now, texting can be cheaper than phone calls, and I realize that it has its times and places. More people have access to text messaging more often than they do to IM. Libraries and public places where it's hard to sneak in a phone call if you have something to tell somebody, or something to ask somebody, or it's not necessary to draw it out into a full phone conversation. You can just send a quick little concise message and not waste expensive cell phone minutes or spend the time clarifying things by voice. You and the recipient can handle a text message at your convenience/when you're prepared to, instead of stopping what you're doing now to take a phone call. Basically, more availability, and more convenient than a phone call for some things. My dad and I tend to use texts when asking each other a simple question, and take it to a phone call when there's a back-and-forth exchange (like one question after the other).
It's a shame customer service is still all about the voice call.
"nine four five four?" "no, five one five four"
"fields?" "no, field - eff eye ee ell dee"
"w-o-o-d-d-a-l-e?" "no, wood space dale"