This thread has got me confused now.
I've created Facebook Timelines at 850x315.
Have the file dimensions changed since to double that? Or is that a creator preference to make the file larger to ease editing and have the buyer resize?
Do not worry. This is just a discussion regarding the loss of quality when these covers are uploaded to facebook. Since many authors say that they've never got any complaint from buyers about quality with 851x315 pixels, you can upload those you've already created.
Though before creating any new templates in future, you can try out these different dimensions listed here and test it out yourself if they work or not. Also try to figure out which is more popular between buyers.
Rest assured, there's nothing wrong with the dimensions you've created them in and you can upload them without hesitation. Although, I think it's 851×315 pixels and not 850. I might be wrong though.