There is several ways to do this on Google. However, there is no one best solution.

The easiest way to do this is using the startup web service called. ifttt. (meaning “If this then that”).
This is still beta service, but works fine and looks quite useful, because this includes lots of SNS all together and ifttt might include new SNS to come up.

ifttt site is so simple and starightforward so you don’t need any extra explanation, just go and use it. For your interest, I put my task setting in ifttt below.

You might think of more useful and interesting combinations.

Unfortunately, ifttt dosen’t support G+ as one of SNS channel to include in tasks.
So, to sync with G+, we need some more googling.

Here are some samples that I applied today.

If you use Chrome, you can use extension that let you share your facebook status with G+ or Twitter. That is “Publish Sync“. But, this is kind of manual way of sync.

If you want to sync from G+ to Facebook automatically, you can use other way.
1. check your mobile facebook update email.
2. register this email as one of circle at G+
3. include the circle when you post at G+

More various ways and detailed instruction can be found here.
And see the second search result. That was good to reference.

Cheers.