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The problem is that dating sites attract a very specific audience, and normal people looking for normal relationships don't stay there for long. They either find what they're looking for or give up and delete their accounts. But those who constantly hang around there are often those who can't find a partner or simply enjoy changing partners like gloves. Therefore, the sample size may be large, but it's not very representative. I know many people who found a partner at school/work, at gatherings based on shared interests, through a common hobby, etc., and very few who found a partner on a dating site.