1. If you have something to tell me, and it's pressing, and it's short, send me a text.
2. If you have something to tell me, and it's pressing, and it's long, send me an email, and then send me a text telling me to check my email.
3. If it's not pressing, send me an email.
Let's look at the various ways you can reach me, in order of how quickly the message gets to me.
SMS: texting gets to me the quickest. The next time I pick up my phone for any reason, your message will be there, and I'll at least glance over it. So if the message has to get to me, and it has to get to me *now*, this is the way to do it.
IM: instant messaging is the second quickest way to get to me, and probably the best way to reach me for a conversation (I'm on gchat and AIM). I'll most likely get your message the next time I'm at a computer screen, and possibly on my phone as well -- so, within an hour, most of the time. Unless it's a conversation I'm absolutely dreading, I'll be game to chat for a while, and often I'm able to return the message immediately.
Facebook Message: this is a special case of IM, and it's a special case that doesn't work very well. Sometimes people send me facebook messages and it works just fine, functioning like the IMs as above. Other times, the messages don't show up in my IM client. Still other times, they don't show up in my IM client *and* I don't get any facebook notifications about them. So generally, treat this medium as spotty and unreliable. And even under the best circumstances, facebook messages are far less convenient than...
Email: I check email on a reliable schedule -- I invariably check it several times a day -- but not a very quick one. I don't have any notifications set for incoming email, so I could go for five or six hours without bothering to check my inbox. That said, if you have something lengthy to tell me, I'd rather receive the message as an email over anything else.
The Phone: like most introverts, I hate the phone. If you call me, and you're not related to me, and I'm not expecting the call, I will probably let it go straight to voicemail. And if you leave me a voicemail, it may be a day or two before I (grudgingly) listen to it. So: if it's an important call, text me ahead of time and let me know that. And if it's possible to make plans for the conversation ahead of time, then I'm more likely to pick up the phone.
That's all I can think of for now. I'll add more as I think of it.