A user profile in iTunes' Ping social network.

Digital music guru and technology analyst Matt Rosoff watched Apple's announcement of its new Ping social network for iTunes today and was reminded of Microsoft's Zune Social music service. I read Rosoff's CNet blog post about Zune's new predicament and was reminded of Classmates.com.

That's because one key in any social network is numbers.

Facebook has been able to leverage that fact, building its user base to overshadow Classmates and older rivals in general-purpose social networking. And Apple likewise has a critical mass of users in its newly launched competition with existing social music services -- enjoying an installed base of 160 million iTunes users around the world, according to Steve Jobs' presentation today.

"Ping will immediately have an addressable user base of more than 100 million vs. a couple million, maybe, for Zune. Regardless of feature comparisons, that's game over," wrote Rosoff, an analyst for Directions on Microsoft in Kirkland, in response to my email inquiry earlier today. "Nice idea, but Microsoft never got enough market share to make it relevant, and Apple doing this now staves off the chance that Windows Phone 7 popularity could have revived it."

Apple's new service will include many of the basics of social networking -- letting iTunes users follow one another, follow artists, create a profile, share information about music they're listening to, and so on. It's available today with an update to iTunes.

A couple caveats: Microsoft has the benefit of offering a subscription music service, the Zune Pass, which opens up more opportunities for sharing and discovering new music with friends. Apple's pay-per-download model doesn't work quite as well in that regard.

At the same time, Apple faces the challenge of persuading people to take take the time to particpate in yet another social network. And Zune is far from the only music-oriented social network or socially oriented music service that grappling with Apple in a new way. MySpace, Rhapsody and many others are on the list. Amazon.com is no doubt paying close attention, as well, as the operator of its own MP3 download service.

But the ironic thing here is that Microsoft was well ahead of Apple in this area -- contrary to the common rap against the Redmond company. Former Microsoft exec J Allard launched the Zune Social, service, with Bill Gates by his side, nearly three years ago.

If Apple ends up making Ping successful, the situation shows that it's OK to be late to a market segment, if you can take advantage of an existing base of users.