What social media do you use for your small business and why?

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I don't, because I'm business-to-business. Big companies who sell primarily to consumers love social media marketing because it's so cheap compared to their normal avenues-- all it really costs is time-- but so far I haven't heard any reliable reports one way or another what effect Twittering (etc.) has on a business's image or revenues. In a small business your time is as valuable as your capital, though, so even if social media are "free" to use it's still time not spent on productive labor or normal marketing (which are known to be effective).