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How do you filter my "own" posts without correlating my Facebook id with a post-author id?

All user associated data uses our own user ids, not facebook's ids. That includes post author ids.

Your system seems to be able to correlate the FB id and user id on login to filter my own posts and aggregate my friends postings.

Am I wrong to assume that this ability might enable de-anonymization of postings after a server compromise?

No, you're correct in that assumption.

In case of a complete server breach, that would indeed allow for de-anonymization of posts.