I'm mainly using Windows Live Spaces because it integrates well with the other Windows Live services that's I'm using on Windows 7.
The same services are available for XP and Vista, too, but I don't use those installs for social networking (XP for spare computer, Vista for HTPC and gaming computer).
I especially like the ability to upload photos to my live space directly from Windows Photo Gallery. It reminds me of the shell-integration tool Imageshack released before they decided they were going to try to go after more advertising dollars by releasing an adware-ridden toolbar for Internet Explorer instead and, subsequently, falling into mediocrity and oblivion.