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 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 and building content 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.