location-based social networking

there's a whole slew of emerging players looking to extend the web2.0 social networking (or whatever) experience with location-based apps and services. while there are differences among them, the various services (all free) listed below share common features ranging from locating to mapping to tagging to sharing. having played with each briefly, i'm hard-pressed to define the unique value-add of any one over any other - in other words, i don't have a preference among them.for whatever it may be worth, while each allows for mobile device access/use, they are all still primarily optimized for the PC experience, and, each requires (naturally) some level of ongoing "active" engagement (input) by the user. "passive presence" and more mobile-optimization will almost certainly come with the introduction and mainstreaming of integrated gps handhelds. until then, while cute and fun, these services are somewhat limited in their location-based promise. mobility, after all, is the ultimate extension of the web2.0 experience - indeed, mobility completes it.