What is the best way to expose data (total users, forum posts, language, etc,.) for external reporting?
I suppose you care most about the import capability of the external reporting tool. Barring knowledge of the external tool, a generic format (CSV, for example) may be safe.
Telligent Community includes downloads (generally CSV) of some data, but not a generic export capability for the purpose of external reporting. Such an export could be generated using the REST API.