If you try to share to Facebook and it fails (even if it says it has been shared successfully) the tips below should help solve your problem and allow you to share.
If you only share to Facebook from the website and don't use the mobile app:
Go to your profile settings and from the "Social Connections" pane on the right-hand side disconnect and reconnect your Facebook account.
If you share to Facebook from the mobile app or use the mobile app at all try one or both of the following*:
Go to your Profile page in the mobile app, log out, close the app completely and then log back in.
iPhone app only - Go to your phone's settings and toggle the Facebook Strava connection. To do this navigate to Settings > Privacy > Facebook > Strava then toggle the switch off then on. This can also be done by going to Settings > Facebook (scroll down quite a bit to find this) > Strava and toggle the connection off then
*After doing either or both of the above, try sharing an activity, if it fails (it may say it has posted but still fail) please try again. Many times it works the second time.
If you're having problems sharing just one specific activity:
Use the Facebook debugger found here: https://developers.facebook.com/tools/debug/. Insert the URL from the activity page for the activity that you'd like to share.