Facebook often runs their own advertisements as sponsored units on the main newsfeed. The ads typically promote new features, job openings, their ad platform, etc. So these below promotions for Facebook jobs and Facebook's Ad Platform are nothing out of the ordinary... but what is noteworthy is how Facebook has now begun using their 500m user milestone as a marketing message:

Advertise on Facebook Promote your business to 500m+ active and enaged users on Facebook. Develop your brand online by connecting with your audience.

Wanted: Ops Engineers! Want to keep Facebook running for over 500m active users? Apply today!

Interestingly, the marketing message isn't too dissimilar to that of the upcoming Facebook movie ("The Social Network"):