If a person throws a rock at another person and injures them, the rock is not sued, the person who throws the rock is sued. If a third person had fabricated the rock, let's say out of concrete, then that third person is not liable, because, for one thing, they have no reasonable way to prevent the main offending party from throwing the rock.
For example, suppose a person were walking up some steps and slipped and injured themselves. It turns out, it was icey, and the person who owns the steps did not put salt on them (a reasonable precaution), in which case the owners of the steps would be responsible, as they have the reasonable ability to prevent an injury, which it is reasonable to expect might happen in icey conditions, and did not do so.
Let us extend that to the internet web site. How can a dating site reasonably protect people from misuse? If there were a reasonable standard to guarantee people's identity through the website then the website could be considered negligent if they did not implement that reasonable and common standard. There currently exists no such reasonable and common was to guarantee a person's identity online, and so the website has no obligation to protect from fraudulent use.