I think you have two (or three) separate questions here.

 - **Which donating service should I use for my blog?** - This is a very broad question that is, basically, asking for a [shopping list](https://communitybuilding.meta.stackexchange.com/questions/1270/shopping-questions-on-community-building) of possible responses. As phrased, it is off topic. However, I can very easily see a question along these lines being focused and made to be on topic. Donations, advertising, sponsorship, etc. are all relevant to building and sustaining some communities.
 
 - **Is asking about the audience issue in blogging on-topic?** - Blogs are a unique snowflake. There are blogs that form communities around them, but there are others that just want readers. You'd have to explain what you mean by "audience issue". [Improving engagement](http://communitybuilding.stackexchange.com/questions/1067/increasing-engagement-on-a-new-gaming-blog) and [building content](http://communitybuilding.stackexchange.com/questions/464/recruiting-guest-content-creators) for a blog is on topic. 
 
 - **If contributing means money or knowledge, then I think my first question is on-topic. But it's not clear for me that any question about blogging will be good here.** - To me, "contributing" means "time, knowledge and/or finances". If the question is about how to improve user contributions and you have a question that is focused enough for us to answer, it is welcome.