I don't see many useful applications of GPT-3. I'm sure 90% of its applications will be spam, fake reviews, SEO, fake news and everything fake in general..
I can imagine all of the growth hacking tactics everyone is going to implement. There's a startup out there already using GPT-3 to automate sales email creation.
I think seeding creativity is rather interesting. Tom Scott's recent video on using GPT-3 to come up with ideas for new videos is a great example.
Reddit was started with fake content and fake users, I think.