Smart Job Board is the first platform provider, to my knowledge, that now offers recurring billing options for employers. They made the announcement on their blog last week.
Recurring revenue is great for job boards (or any internet business). Helps to maintain steady income so think about ways to integrate it into your product mix. Also save you time from having to collect or sell to customers. You can choose to bill your customers on a monthly or yearly basis.
There is one more type of revenue model that might make sense for software vendors to add. A membership based model where you charge job seekers and make postings for employers free. Would only make sense to use in certain industries but might be a good source of revenue for platform providers. For now you could try using membership software like Memberium.