In larger orgs there's usually a social strategist [it's even more fun than the title implies] who creates a page often under a fake account tied to the company's email [Facebook does create "advertiser" accounts without personal profiles]. From there a tool like CoTweet is linked up and contributors, whether they be in marketing, support, pr or an agency are given access rights through that system. The system logs actions, allows for review, case management, scheduling and all sorts of other fun controls.
But because of EdgeRank's [assumed] negative impact to API postings, many teams also have a few people with direct access who make the actual posts via the web or a mobile interface [and yes, this often means personal profiles are still used]. The larger the team, the higher level the role but ultimately anyone with access is a danger so you either trust someone and bring them, or don't bother playing in the space.
Coke is an interesting case as their page was started by a few fans who are, or were, still involved. Fun reading if you google the topic.