Did you know the average life of a Facebook post is 3 hours and 7 minutes? After that, you don’t lose much by posting again, but posting before the life of your post is up can cut short its lifetime and overshadow it. It doesn’t mean you should post every 3 hours, as 2-3 posts a day is enough. http://techcrunch.com/2012/01/17/how-often-should-facebook-pages-post/
When Tweeting, you probably can’t suffer from tweeting too much if you are posting relevant info/links, retweeting others and having conversations, i.e. providing valuable content. http://todaymade.com/blog/how-often-should-i-tweet/ Being a part of the conversation is one of the most important aspects of being on Twitter.
Google+ posting I think falls a little in the middle. More than Facebook is fine, but I’d lean more toward 5 times/day than 10 if you are providing valuable and diverse content. Share stuff from people in your circles. Google+ is growing and plays an important part in the search ranking algorithm. In the past 6 months, Google+ has gained hundreds of millions of users. It shouldn’t be overlooked. http://www.computerworld.com/s/article/9243634/Google_popularity_jumps_with_540M_monthly_users