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High switching costs + nothing compelling enough to move to a competitor for.

I know of at least one coming with another rumored to be in development. Without the cutesy factor of Plurk.

Plurk (www.plurk.com) funky UI, so a slightly higher learning curve, but pretty interesting.

I'm currently only using Twitter as a sideblog, and Plurk's version seems quite obtrusive. I don't understand sites' obsessions with branding their widgets.