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SEO in stackoverflow

Stackoverflow pages seem to do ever so well on Google - i.e. come up very high. Several times things like this have happened to me:

• Google to search for answer to question before thinking of using SO. Don't like the results that come up. Ask on SO. Keep Googling anyway in the background. SO page starts appearing high up on Google results.
• Encounter a quotation in an SO answer. Google it. SO answer just seen is the top result.

Why / how does it do so well?