The title basically says everything about the problem with the Chrome plugin for multiple SNS account management. One of the reviews for the Chrome plugin mentions a solution for Windows machines here.

Under Mac OSX, the fix is essentially the same, only that the wrong popup.js file is located at

/Users/yourid/Library/Application Support/Google/Chrome/Default/Extensions/aicelmgbddfgmpieedjiggifabdpcnln/2012.1.21.0_0

A working copy of the popup.js file can be downloaded here.