Email is only a good platform inasmuch as you can only contact people you know by email (or that have given you their contact info).
Facebook has its dangers. If someone does not link to or friend heretics, etc. then it can probably be a good platform - just have your settings changed so that anyone can read your posts. When they search for Bible topics, they find your posts. You can modify your settings that no one or those you choose can reply so you don't get flooded with spam or evil people trying to undermine your messages.
The biggest danger to me is the connections I see between people and other ungodly people or organizations - ie. someone might be a sound Christian but then have ungodly people friended and linked to them. You go to check out related people/links and you get immorality, immodesty, ungodly music and entertainment, in your face. Having a link to a godly preacher or ministry, with no links to sinful material or compromising individuals/ministries, I can see it being beneficial.