There isnt any exact API to get the hashtag posts,you may try using search API with q=%23hashtag
https://graph.facebook.com/search?q=%23hashtag&type=post&access_token={ACCESS_TOKEN}
For getting wall posts and newsfeeds
/user/home The user's news feed.
https://graph.facebook.com/me/home?q=%23hashtag&type=post&access_token={ACCESS_TOKEN}
/user/feed The user's wall.
https://graph.facebook.com/me/feed?q=%23hashtag&type=post&access_token={ACCESS_TOKEN}
Permissions :read_stream