So I am a US citizen and I have a Chinese L visa for multiple entries that was issued in June 2012, the enter before date was June 2013. I am planning to travel to China in a month (August) and was wondering if my visa is still valid or I should apply for a new one. Thanks!
Your visa is expired and you should apply for a new one.
The enter before-date is what comes closest to the expire date, the meaning, quite obviously being that you must enter before the date given.
In a comment you mention the 10 year visa. While such a visa is available to US citizens, the respective agreement has only been put up in November 2014 so we can exclude that your visa issued in 2012 is a 10 year visa. You can recognize a Chinese 10 year visa from the issue-date to the enter before-date spanning 10 years.