You probably think instant messaging (IM) as something teenagers do to chat up friends online, whether across the street or across the world. But IM has some buttoned-down business benefits, as well as some risks you may not be aware of. Unlike e-mail, in which you fire off messages to recipients who read them when they next check their in-box, with IM you and your recipient have to be online at the same time. But the messaging is instantaneous, or nearly so, and rapidly interactive, back and forth. Many business people use IM for collaboration. Among other things, it lets users send spreadsheets or other documents as attachments and do voice and video conferencing. An estimated 21 million business people worldwide use IM, and though this figure is roughly 10 percent of the total number of IM users, using IM for business is expected to grow much more quickly over the next three years than consumer use, according to figures by Robert Mahowald, research manager at IDC, a market research firm in Framingham, Mass.