When we hear the word "prophecy," most of us immediately think of one of two things: the Book of Revelation or psychics. On that score, one is from God and one is not, but both seek to tell us about future events, more or less. That's the most common take on the definition of prophecy: speech about future happenings.
Biblical prophecy, though, features another definition in addition to the first: a sharp critique of a society serving as a call to return to God to avoid future judgment. Think about the prophetic oracles of Isaiah, Jeremiah, Amos, Malachi, and the others. They all spoke words straight from God as rebukes and warnings. To that end, as Paul writes, the gift of prophecy and the role of the prophet continues (Ephesians 4:11).
Modern-day prophets seem to find a home on social media sites. I recently severely curbed my own usage of Facebook to avoid the endless political posts, but on every one of those posts (and a dozen other types), a Christian comments to tell people how to act as a Christian in the given situation. We need those voices to call others into accountability and the will of God. The Church needs people to say "Here I am; send me" even on Facebook -- perhaps especially on Facebook. But those voices must be ones of love as well. Our words are worthless without love behind them (1 Corinthians 13). Love doesn't stir up anger deliberately, nor is it arrogant. But it does correct. It chastens. It comes from God.
If you have received the gift of prophecy, prophesy. Be a Facebook prophet if God calls you there. Just don't be cruel in the name of God. Don't believe you alone can speak for the Almighty. Be a prophet in love.