Facebook offers an iCal link that you can add to your Google Calendar to get alerts about upcoming events right on it.
The link is available from any event that you have joined. It is available an option named Export Event from a dropdown at the top right of the event’s page.
You would get the link right there in the popup that appears. Just click on it to add it to your Google Calendar account.
You can also copy the link and add it to any app that supports such iCal feeds.