Email works great if you have an established and targeted mailing list. A company hired me a while back to get them a more engaged following, we sent 40k emails and got over 500 likes after subtracting internals. They were ~$200M company, so they had a real list. If its a spam list I doubt this would work.
Just adding the facebook logo to stuff like business cards, packaging, etc. can do wonders as well. For one of my local clients we just put business cards and a QR code in their lobby "follow us". People get bored waiting, we were getting a couple likes a day just doing that. If that's not working well, have the receptionist hand them out to each person as they come in. That always works.
But if you have to, running ads can be a cost-effective way of building a FB following. Its just important that you target them correctly, otherwise you're wasting a lot of time and some money. If you know what you're doing they should convert <$1/ targeted like. I don't like to do this because it takes a lot of time, but if you have time it will work.