More description for enricocantori's solution.

Please try this small CSS code. Put it in Dashboard > Forums > Settings > Styles > Custom CSS Code Textarea. Save it delete all caches, do Ctrl+F5 on forum front-end. Then test it on other mobile devices. Because mobile browsers have a hard cache, you’ll not be able see the change immediately.

.fb_iframe_widget iframe {
      position: relative!important;
}

Also please don’t use Private Session of mobile browsers. Facebook doesn’t allow use Login Button on private sessions, it doesn’t allow use Cookies so make sure you use regular (default) mode of mobile browser.

.fb_iframe_widget iframe {
position: relative!important;
}

Hi Robert,
Do you have a cost idea when you do this for us?

I'm sorry but I don't follow you. What should I do for you? I've already mentioned, that we don't support issues in Facebook API configurations. You should make sure all is setup correctly, like it described here:

https://wpforo.com/community/faq/how-to-get-facebook-app-id-and-secret-key/

these are share tags and don't have any relation to wpForo FB social login button. I recommend install a brand new WP with wpForo on some subDomain or subDirectory and configure a new FB login APP, so you can see the difference and find the problem maker.

caution-solidWarnings That Should Be Fixed
Inferred Property
The 'og: image' property should be explicitly provided, even if a value can be found in other tags.
Missing Properties
The following required properties are missing: og: type, og: image, fb: app_id

---- ---- ---- ----

When we get the following URL through the debugger we get to see this:

caution-solidWarnings That Should Be Fixed
Inferred Property
The 'og: image' property should be explicitly provided, even if a value can be found in other tags.
Missing Properties
The following required properties are missing: og: type, og: image, fb: app_id

https://goo.gl/UiJCto