I'm sure that strategies exist on any platform. Zynga doesn't seem worried that Facebook will copy its games (even if they do hate paying the platform tax). Many products are pretty safe from Facebook. For example, build an app for dentists, or personal trainers, or pregnant women.
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...,