There's more to it than preference, fad and fashion. Tools get replaced when a better one comes along that transmits messages that exist near the same point in { Fidelity, Latency } space. Telephone calls are pretty high fidelity, zero latency (if they're answered). Snail mail is for high fidelity, high-latency messages (like phone bills). Nobody would snail-mail a lunch invitation any more - too high a latency. And so on. Twitter is lower latency than IM; IM is lower than email. They vary in fidelity (Twitter's 120-char limit). There remain room for a whole menagerie of tools in between, above and below. And these will be eclipsed when higher-fidelity replacements get invented. Btw somebody explain to me why its ok to automatically switch a phone call (low-latency, high-fidelity) to voice mail (abysmal latency, half-duplex crap fidelity)? This really bugs me. I don't set up voice mail because its absolutely the wrong choice - send me an email if you can't get me on the phone, or an IM or pretty much ANYTHING but voice mail.