1. What's the source of the data?
2. How is the extrapolation done? ("If one were to extrapolate...")
3. The headline (12% drop in the last week) contradicts the body of the post (60% of a 12% difference in the last week).
It would not surprise me in the slightest if Stack Overflow was becoming less popular. However, without more details the post comes across as just pulling numbers out of thin air.