For 2 years, Wikipedia ran a collaboration of the week.
Each week, users would nominate an article (or several articles) for the group to dramatically improve within a short amount of time. Research shows this was quite successful at engaging users.
For more mature communities, a similar approach could be used. You can highlight a challenge your industry/sector faces (or even a common problem individuals face) and set a week-long task to tackle it.
Create a specific thread or location for members to work together. Follow progress on daily news posts. See how it works.
The benefits includes more members participating, more interactions from each member, and valuable outputs from the goal itself.
p.s. We now have a jobs page up. Click here.