Here's a thought experiment for you. (See what people are saying: google mobile identity , twitter search mobile identity )
You're waiting to meet someone who you don't know very well, and you have a few minutes with your cell phone to do a little bit of searching or browsing to see what might be useful to know. If you have one social mobile site to look at, which would it be? And when the meeting is over, where do you jot down that note to self to remind you for the next visit?
Mobile facebook. If you're frends already, this works pretty well, as long as your correspondent is keeping up appearances. This is pretty much an all or nothing question.
Mobile Google search. This works pretty well if the person you're talking to has a distinguished name, but not everyone does. The results are an absolute mixed bag and vary wildly depending on how much personal SEO someone does and how lucky you are with Google mobile-izing the results.
Mobile LinkedIn. Dry as dust, this won't even get you someone's picture so that you know you have the right person; suitable for background employment history, so I guess if you're doing impromptu interviews it might be a good part of your mobile arsenal.
Mobile Twitter. That person you're about to meet, did they have a peanut butter sandwich for dinner last night, so you can avoid suggesting one again? This is perhaps the most open ended resource to tap into if someone is on Twitter, since it's plausible to believe that you might be able to get a message through that they'd see.
Mobile Gmail search. If you have had a bunch of correspondence with someone and you use Gmail, one plausible tactic is simply to plow through your old inbox and unearth some conversation from the past to bring up again. Results may vary but it's a much more personal search than any of the others.
Mobile Flickr. Is your friend a photographer? Look at their photo stream, it's like a personal channel into their life. This only works if you know there's photos to see.
Mobile wiki. If you, like me, are one of the odd people who keeps a personal wiki, then you can search through it and see if there's some wiki note-to-self that you wrote that triggers a memory or association. There's a variety of similar memory augmentation tools that fill a niche that was once called a "personal information manager", and the right ones of these offer a jumping off point for reflection or memory or jotting down a few notes.
Mobile address book. If your address book on your mobile phone or web-based call manager has a notes field, you might have written down a few things or a lot of things that you want to remember for that next conversation. Most of these that I've seen are pathetic fixed-length field plain text databases with no inter-contact linking and very weak group management, but you might have worked around something's limitations to make it your own.
The overall complaint I have about all of these is that nothing does an awesome job of being the everything-spot to look up a few notes before an impromptu meeting and to jot down a few notes afterwards. As a rule, the web-based stuff is weak on being a launching point for phone calls, the phone-based stuff is weak on web services, and the social networks aren't private enough. At some point you just want to carry around a little black address book and write things discreetly in pencil.