The original intent was to flag controversial conservative views. So obviously when conservatives see that "fake news " icon, they flock to Those articles, as easy identifiers for the type of news they prefer.
Regardless, this doesn't dismiss the fact that FB still has and always had a bias in selecting articles for flagging. they still think they are the ultimate arbiters of truth and opinions. It's one thing if FB themselves write news article, thus can pander their ideology as they see fit. However, they are merely a platform for other news sources. Thus this fake news identification is inherently partial and judgmental on opinions that oppose FB ideology.
Which again anything that FB opposes will automatically be confirmation for values of opponents. Thus anything FB tries to curb 'truthiness' will at best be ineffective, at worst drive even more readers to flagged articles.