400 Bad Request
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.
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.