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How are the SE questions that are used in the SO sidebar ads chosen?

If they have a more proper name than 'sidebar questions' please let me know and I'll edit; but I'm referring to the questions from other sites that sometimes appear on the right sidebar of the screen when you're asking a question on SO. Here's one:

sidebar question example

I'm wondering what the criteria for their selection are? Sometimes they're really popular questions, sometimes they're odd questions by new users that actually don't have very good feedback in the comments, sometimes they're just regular-old-joe questions. There doesn't seem to be much of a pattern I can detect between them, except of course that they're trying to direct you to other sites in the Stack Exchange network. So I'm curious, does anyone know how these questions are selected?

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