400 Bad Request

There are different reasons why you could get 400 Bad Request error.

Incorrect Tokens

Check that you generated tokens properly and copied/pasted the whole thing.  Authentication Guides

Facebook Token is expired

User access tokens come in two forms: short-lived tokens and long-lived tokens. Short-lived tokens usually have a lifetime of about an hour or two, while long-lived tokens usually have a lifetime of about 60 days.

Basically if people visits your website regularly Flow-Flow extends your token automatically. So you don’t need to take any additional actions. Otherwise you need to check that your token is live and re-generate it if necessary.

Regenerate or extend Facebook token

Incorrect feed configuration

Check spelling and notes while editing feed under the  Content to show field. For example you probably added @ symbol which is not needed. Or you added account name that doesn’t exist etc.

Social network API requests limit exceeded

Try to decrease  Load Last value on Stream settings. Also check that feeds are properly configured and you use correct tokens.