No i don't think so it will effect repute of facebook because it's not a big thing to say facebook is copying twitter although facebook have many unique and different things if you compared with twitter.
Facebook is a treasure chest of information and it was always hard to search for trending topics (the way Twitter does). The hashtag will just make it easier to see what's going on in the world as well as locally with your immediate friends.
I don't think it effects Facebook Reputation because Facebook is totally different from Twitter and it is more user-friendly as compare to Twitter. And also it is a good for us, So we can target our keywords in hash tags.