[Helen Newman] “The reason I’m thinking this is a FCPX problem is because I NEVER had the problem when exporting from FCP 7.”
You’re using the same computer and monitor with FCPX that you used with FCP7?
While FCP7 was Quicktime Frameworks based, FCPX is CoreMedia based. If anything FCPX/CoreMedia seems to be better.
To do your own Quality Control you should check your encodes before uploading. If they look good then YouTube and Vimeo are the culprits.
Of course you’re checking all this on the same computer with the same calibrated monitor right?
If you find a consistent pattern then you should add post processing filters for your encodes (Add Video Effect) in Compressor otherwise you can duplicate your timeline in FCPX and do a separate grade (possibly with an “adjustment layer”) as a YouTube or Vimeo specific version.