I had the pleasure of a trip to Boston to participate in a panel with some some amazing folks. Led by Sean Lindsey, and with Raffi Krikorian from Twitter and Praveen Alavilli from Paypal, we discussed a lot of the issues around running and using "platforms."
A large part of the initial disucssion was around what a platform is. Essentially you are either outsourcing a hard technical problem — such as GeoLocation or Payments (Yahoo! Geo, PayPal) — or you are getting access to a large community of users (e.g., Twitter, Yahoo! Status, and so on).
Often you get both, such as with PayPal: Not only do you get access to the payment API, but you also get a large community of people who already have PayPal logins.
One interesting part of the debate centered around how to manage multiple platforms, specifically with regards to reliability. All the panelists agreed it was a good thing not to put all your eggs in one basket. Use multiple platforms, both from an infrastructure point of view — like using Amazon EC2 and Rackspace — but also from a user perspective.
Why wouldn't you want to share your application with Twitter's 180 million users and Yahoo!'s 600+ million users? Why pick one or the other when you can pick both?
PayPal recorded the session and posted it on YouTube. You can check out the playlist or start watching.