Flutter Web - In App WebView or Browser wont show properly certain website like facebook and google

I want to create all-in-one messaging on flutter web App. I just want to compile all social media apps like messenger, WhatsApp and Hangouts chat in one dashboard. What I'm thinking is using a webView that will display the site on my web app both browser and mobile via URL Link. I integrate via the flutter web browser plugin and it can successfully show flutter.dev website, but not Facebook or Google. It says "Facebook or Google refused connection". Also if you can suggest a better way to create this app? thank you

Facebook refused connection Flutter WebView Code