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There is no difference between a free or premium domain, at least Google does not evaluate the domain itself, what Google evaluates is the profile of the domain, backlinks, internal links, content, relevance and so on. So usually free domain has no value from the Google perspective...why? who will invest money, time, links and content on a free domain, I mean if you have the money to pay for hosting, content, and links why you will use a free domain....does not make sense. How many Blogspot sites, free WordPress.com websites do you know that have real value and authority. That's why I think, .cf / .tk / .gq / .ml / .ga probably will not have any value for linkbuilding.