Do services social monitoring tools like Mention.com, Socialmention.com, and Nitrogram rely soley on APIs or do they crawl or cache the meta data?

I was researching on social monitoring tools like Mention.com, Socialmention.com, and Nitrogram (http://nitrogr.am/) and noticed they leverage many different social media platforms to search for keywords.

From an engineering perspective, do they simply just poll realtime information via the platform APIs, or do they cache/crawl the data in order to speed up searching and indexing? There are probably various levels of this, but wanted to just seek insights from a best-practices perspective.