Are you sure it is using ram for apps or is it using it for the system. If there is ram available it will use it for other tasks, often it will use it all, but when you start an app it will return the ram.
Look in the activity monitor, if it all appears to used in the 'cached files' section, then it is normal as above.
A good explanation of what is likely happening can be found here:
Yes, it's Linux, but the same thing.