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Trendspottr. They were the first major player in this space I could remember. Their API can be found here https://trendspottr.com/api/

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Yeah, I second TrendSpottr. They not only have an API for surfacing trends for any topic or keyword but also an API for trending alerts. http://trendspottr.com/api. I also use their trends dashboard Signal http://trendspottr.com/signal.

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