Like any other resource or dependency that you build into your business, using a platform requires an ROI calculation of some sort. The distribution advantage needs to be worth the risk.
The risk is not very hard to assess. It's pretty binary per startup opportunity. Do the relevant incumbents' revenue models, distribution models, or inherent cultural limitations prevent them from competing with you. @paulsutter covered the first two well early in this discussion. For the last, Apple has great difficulty with social, Facebook hasn't figured out Touch, Google is excessively focused on those two competitors, etc.
Please also see these two comments: http://news.ycombinator.com/item?id=4336783 http://rafer.net/post/28638883246/mark-i-know-for-a-fact-tha...,