The API methods /users/{id}/network-activity and /me/network-activity, both return a summary of a user's activity across the Stack Exchange network and do not take a site parameter.
They are currently listed under Per-site methods → Users; they should be listed under Network Methods → Users instead, just like /users/{ids}/associated and /users/{ids}/merges.
The sub-header clearly states the difference between:
Per-Site Methods
Each of these methods operates on a single site at a time, identified by the site parameter. This parameter can be the full domain name (ie. "stackoverflow.com"), or a short form identified by api_site_parameter on the site object.
and
Network Methods
These methods return data across the entire Stack Exchange network of sites. Accordingly, you do not pass a site parameter to them.
