Explore FAQ #4:

Why are some Explore analytics only available for Twitter? 

The information available for each social channel will vary due to each social channel API and terms of use.

For example, Facebook supports searching for content within a pre-defined list of Facebook pages while Twitter supports searching against all public tweets. Because Meltwater has such in-depth data available for Twitter, we have created the Twitter Insights Tab within Explore to surface detailed analytics and trends on Twitter. For more information about Twitter data in Explore. 

To learn more - review the Help Center article Get to Know Explore Widgets - Twitter Insights Tab