I think it's not danger to democracy. Its dangerous to their view of democracy. Facebook is a free platform on which everyone can express their opinion. So facebook is a chance for the parties and politicians who aren't as financially powerful to make an expensive campaign, to advertise themselves. For example, in an election a few years back I wasn't sure what to do. I didn't want to vote for  the 2 biggest parties, so I was thinking boycott, but I saw a debate on Facebook, and a politician from a smaller party was making some points I really liked, and I voted for them. They did not won, but from that moment they are gaining more and more fans.