Here I am only talking about measuring registered users, please do not suggest Google Analytics
I also want to know how can I measure avg. monthly hour spend by users?
You get visitor level information on how active they are etc.
I'd recommend KissMetrics. it allows you to track events on individual level. So you will know for each user how much time he has spend on the platform, what he has done exactly and how he is using it.
MixPanel is also a good option and they are quite similiar with some little differences.
Instead of spending more time and money on other products, if you have Google Analytics running on your site already. All you need to oo is add a custom variable say for example logginein: True/False and then you can easily filter for active users.
Let me know if you need help.