Here’s how I see the forms of communication and what they say about who’s sending the message:
Twitter or forum message to the public: “I need help, and I’ll take it from anybody who’s available, inside or outside of my company.”
Email: “I need you specifically, but it’s not urgent.”
Instant Message: “I need you specifically, and whatever you’re doing on your computer right now is less important than what I need.”
Phone Call: “I need you specifically, and whatever you’re doing on your computer or in your office right now is less important than what I need.”
Before you send an IM or make a phone call, stop to consider the person on the other end.
Is what you’re asking for really a higher priority than whatever they’ve got open on the computer and who’s standing in front of them?
If not, be a nice guy: send an email, or even better, ask more people at once so that someone with free time can respond faster.