It's assumed that Facebook's main rival is Myspace. Facebook, with 23m unique visitors in the US in April, according to Comscore, is the only social network that comes within spitting distance of Myspace's 67m. And both gigantic sites took off when adopted by young audiences, Myspace by music fans, and Facebook by college students. But Facebook's immediate challenge is not to Myspace, but to another social network, which might surprise you: Linked In.

On the surface, the two couldn't be any more different: Mark Zuckerberg's social network has only recently expanded beyond college students; and Linked In is a tool for recruiters, job seekers, sales execs and other corporate drones. But Facebook's interface is sufficiently clean, and efficient, that it's attracting a new class of user, the professional.

Browsing through my feeds a few minutes ago, I came across a plea by Scott Heiferman, founder of Meetup, who has obviously been barraged by invitations to connect from Facebook users. "No, I really don't want to spend hours moving from LinkedIn to Facebook," he writes.

The central truth of social networks is that they are only as valuable as the people that join them. By that measure, Facebook will soon overtake Linked In, at least in my world. Since I joined, a month ago, I've had 38 invitations to connect on Facebook, compared with 17 approaches from Linked In users. (Charted, above.) Some of that gap may be to do with my longer tenure on Linked In: many people who want to connect, already have.

But Facebook invitations seem to be, at least for the moment, of a higher quality: people I might want to list. And, because it's a general-purpose networking tool, rather than simply for the purpose of business, it feels less cheesy. We're getting everybody at Gawker Media to set up Facebook accounts, if they don't already have one, and migrating our staff directory to the system. I would have been mocked for using Linked In for that purpose.