It's hard to get a definite answer to this as these companies will not tell us how their algorithms work. Having said that: Based on my experience working with these APIs, it is totally possible to implement a real-time search like "socialmention.com" purely based on the APIs from Google, Twitter, Yahoo, or Facebook. On the other hand, crawling and saving all that data just for the eventuality that a customer might search for it is probably not economically viable and will also violate the Terms of Services of most of these APIs. Bottom line is: They are probably not crawling and caching.