On the internet, you don't need Polyjuice Potion to pretend to be someone you're not. That means that it can be easy for people to fake their identity.
A few months ago, one fan travelled to England from abroad because she thought she'd been communicating with Dan Radcliffe over the internet. In fact, she'd been talking to a fake.
So if you think you've been reading a blog about what Katie's been up to or communicating with her, unfortunately the person you've been talking to is an imposter.