Advice I have received on queries includes listing your writing credentials. As an unpublished author, I was told that I should list my journalism studies and experience as part of my personal bio. Recently my YA speculative fiction was voted as a "Top Pick" for June out of more than 25,000 pieces of submitted work on a large publishing company's international YA writing community site. As a result my manuscript is being reviewed by an editor and they will also consider it for publication, though I realise being an Australian writer with a story set in Australia my chances are pretty slim.
There has been some debate between my writing friends as to whether having a piece making "Top Pick" status is something that should be included in a query letter. On one hand my work had hundreds of extremely positive comments and votes from readers who are in my target demographic, many of whom say they would like to be able to buy my story. On the other hand I have been told agents would not view this news favourably as it is theoretically possible for writers to make that status by clever promotion over amazing writing.
Should I mention in my letters my success of the site and the fact that this company is currently reviewing my manuscript?
Yes, you should mention it, for no other reason than it tells the agent/publisher that someone else is looking at your manuscript.
As for the favourable comments on a blog or posting of a piece of writing - I suspect these will shortly become the online equivalent of 'my mum and my best friend think it's great' (or variants thereof), which is a line that turns up in submissions with surprising regularity and always makes me want to write back and say, 'OH REALLY? Are your mother and best friend going to buy 10 000 copies of your book?' So if you include that information, do it with the knowledge that not much weight may be given to it.
When writing your query letter you should include any writing credentials that you think are relevant, even if it's a short story published in an obscure journal, and so long as the credits don't take up most of the letter - if you have a lot of credits, pick out the highlights. The credits show that you've been writing for a while, that you're trying to find an audience, that you may have succeeded in finding audiences.
However, those of you who don't have writing credits, don't fret - the decision about whether or not to take on an author and their manuscript always comes down to the writing and the story in the end. You may have no writing credits and an amazing manuscript - the absence of credits is not going to make the manuscript any less amazing.