Unfortunately, it's no longer possible to track or monitor keywords or hashtags via the Public Search of Facebook. Since April 2015 Facebook has decided to no longer support keyword searches for Public Statuses in their new API version. External tools such as CX Social and other third party applications that use the API to retrieve data are no longer able to collect public mentions based on a keyword search. Facebook made this decision in order to protect the users's privacy. If you want to monitor brand pages other than yours, you can add these as a stream which will allow you to retrieve the posts and comments from that page.
Here's a full article explaining the changes Facebook made to their API:
If you have further questions, don't hesitate to contact us.