I think there is a need for more social analysis of all these technologies and the possible long term consequences. Most people have a very limited view of the world and technology is supposed to do the opposite by opening up avenues that we wouldn't have explored because of high entry barriers and whatnot. A malleable social graph seems to be a step in the wrong direction. My view of things is limited and myopic and I don't want emerging technology reinforcing that habit by actually adapting to my current habits and regurgitating it back to me.
I could be totally off with this analogy, but this argument is similar to the thinking that overtook my mind a few months ago when I was doing a case study on whether or not Apple should release an eBook reader (complete coincidence).
I kept thinking to myself, "Why would anyone ever buy an eBook reader? Netbooks are becoming cheaper and cheaper (much cheaper than readers even then), smartphones are becoming better and better - and those are full-fledged computers."
To this day, I still don't completely know the answer....except what I do know is that a lot of people spend a lot of money on eBook readers.
Perhaps it's something along the lines of facebook chat - just because facebook has it, and ALL your facebook friends are on it doesn't mean people have switched over to it en masse. I still use and prefer many other chat programs - perhaps for stylistic, habitual, or illogical reasons - but significant reasons enough.
To foursquare - I say keep going. At the very least, competition is good, and at best, I think you have a good shot.
I'm finding the concept of "malleable social graphs" in this article fits well with my desires for the next advances in social networking applications. Food for thought.