one issue with node.js is that it is so early in the lifecycle of the product that there aren't a lot of 'clear winners' in the add-on modules department. it's hard to tell which will be maintained in the long term and which are just hobby projects created so that the developer could learn node.

i would guess that mongoose is your best bet because it was written by a commercial company that is using it in their commercial product (learnboost).

you've probably already considered this, but i would consider google app engine. no database configuration necessary and i watched a video from the 2009 google io conference the other day that showed how insanely easy is it to do things like social graphs and follower using their datastore (assuming each person has 5,000 or fewer friends -- a reasonable number for 99% of the companies out there).