Take the http://www.blackhatworld.com/blackhat-seo/black-hat-seo/341339-free-current-web2-0-blog-list-60-websites-7.html.
Take your niche keywords. Lets assume for a moment you were selling "seo services".
Go to google and, one by one, do a site:web20domain.com "seo services" and see how many results they have indexed.
for instance
site

nsugar.com "seo services" = 13200 results (pretty good)
site:livejournal.com "seo services" = 352k results (wow)
site:blog.ca "seo services" ... hmm only 262 results. That's not very high, so I look at the results, all are 2011. I see one dated 2008, check it out http://outsourcingseoservicesindia.blog.ca/ (page not found) ... obviously blog.ca doesn't like that particular niche to be advertised, best to stay away.
If you're in a naughty niche like escorts, sex toys, adult dating, male enhancement pills, etc. ... best to see what is already indexed on any particular web2.0 before you try posting there, you can save yourself a lot of time.