Re: "If Facebook can control millions of votes [...], is that still democracy?"
Readers of mainstream media tend to flock to a paper that reflects their own views and expect a degree of bias anyway.
For example if you read The Guardian, you probably vote Labour or Green. If you read The Times, you probably vote Tory, if you read The Telegraph, you vote Tory or UKIP.
People who get their news through Facebook are less likely to have an interest in politics and are likely to be easily swayed one way or the other.
Trend a load of articles about, say, how mean the tories are, and those people will vote against them. Trend a load of articles about how profligate Labour are and people will vote against them. Etc