I would hire someone because I already have!
I hired someone to run my facebook because that is what he specializes in. He has alot more time then me to contact people that would be interested in joining our forum. After he talk to them about the forum, he asks them to join our Facebook page to keep in touch with us.
Would you hire someone to manage your Facebook page? I know some companies that do? They hire a person who they think can write catchy statuses and such. This is not something that I would want to pay someone to do though.