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Sites that used to rank well in Google.com have dropped where the only difference between them and sites that did not drop is the ownership of the domain is not in the USA. The hosting is, the registrar is, the subject and content is USA - just the owner does not have a USA address.
It has been widely speculated that Google uses domain info to identify network of sites linked together. Perhaps they also use that same domain info in the algo now to help determine the ranking of sites ...
Another thing that adds to my suspicion... normally Google would give websites hosted in a specific country a "boost" in that country's version of Google. So, if a website is hosted with a Canadian web hosting company it receives a boost in Google.ca and is unaffected in Google.com.
One of the things I am seeing now is some websites (domains) that are registered with a Canadian address but hosted in the US and dealing with US topics getting that boost in Google.ca and several pages back in Google.com
So, regardless of hosting it seems that just the registrant having a Canadian address is helpful in .ca
Actually it could be worded slightly differently where the same site is depressed in Google.com for having a Canadian registrant and unaffected in Google.ca - as that more accurately reflects the reality. The rankings in .com have gone down while the rankings in .ca have remained the same.
Too late now but I think it would have been great to have introduced a .us and .com tld in the early days.
That way we could have the world wide results and also US specific - all other countries seem to have a country specific option except the US. For instance if you are in the UK, you can search on google.co.uk for UK results or "all of the web" - nobody thinks of it as US results yet in the states people often think of .com as US results because there is no specific filter to achieve otherwise.