See my answer here on how you can use the Search API for hashtags:
Need help on employing Graph Search parameters for hashtag query on facebook
Basically, you can call
https://graph.facebook.com/search?q=%23selfie&type=post&access_token={user_access_token}
Be aware that you have to use v1.0 of the Graph API, because in v2.0 searching for public posts will no longer be possible (https://developers.facebook.com/docs/apps/upgrading#upgrading_v2_0_graph_api):
Public post search is no longer available. (/search?type=post&q=foobar)
Graph API v1.0 will only be available until 30th of April, 2015.