Thanks for letting me know that you enjoyed David's guest post. I write about SEO and social media myself but I particularly found Dave's take on it interesting. I'm hoping he'll write for my blog again.
Ideally, the blogs that we comment on should target our niche customer. Having said that, in my case, I help small business owners. So, I comment on a wide variety of blogs. Because, in many cases, it's the blogger who is my potential customer.