My general criteria for determining where I advertise:
1. Does the site seem to spend more time marketing to authors than readers? This is a big deal. Only a very small portion of readers care about the nuts and bolts of publishing. If a site is predominately focused on information for authors, it isn't attracting readers. If it isn't attracting readers, then any money spent on it is being wasted. A lot of these sites proclaim to have huge mailing lists, but those mailing lists are predominately authors who signed up for the mailing list as a requirement of advertising on the site. You are, in effect, spending money to promote to your competitors. And yes, I get the "authors are readers, too" mantra. But authors going to these sites are not as a whole looking for books to buy. They are looking to sell their own books. There is a huge difference in the motivations of the audience.
2. Does the site seem to focus on free/discount books? The problem with these sites is that they tend to be heavily focused on romance and YA books, because those demographics tend to be voracious readers BUT won't spend a lot of money on books. They tend to always be looking for free/cheap books to feed their fix, but they aren't necessarily author loyal because they also tend to have enormous TBR lists. Which means they may download your free book now and not even read it for four months. There is also a growing sentiment in this demographic to simply "wait" for an author's next book to go on sale before buying, which means it is becoming increasingly difficult to sell books in these genres and to this demographic at a non-bargain basement price. I'm not saying you can't make money with this group (obviously, people do!) BUT, if you aren't pacing your release schedule and ready to always price your books a certain way, marketing on these sites will be a net loss.
3. Is the site genre-specific? These are the sites I tend to look for and market on, because these readers have a vested interest in looking for books that cater to their desired genre. Visitors to genre-specific sites are less price sensitive and more willing to invest in a non-free/non-hyper-discounted book if it looks interesting. Particularly because most retail sites are poor at providing good search functionality for most genres. Genre-specific sites tend to have smaller audiences, but those audiences are willing to spend more on a book. Visitors to these sites are generally people who actively look for the type of book I write, and not just people looking for free stuff in general.
4. Does the site focus on self-publishing? I tend to avoid promoting on these sites because most readers don't actively look for self-published books. We are well beyond the novelty phase of self-publishing. People don't care anymore how a book is produced so long as it is well written and available where they want to buy it. So spending money on sites that specifically talk about self-publishing is only going to reach people who actively care about self-publishing, which will generally be authors. (see #1). Note, I differentiate between sites that fixate on "self-publishing" and those that promote a more organic and inclusive indie publishing (including small presses, micro-presses, boutique presses, etc). Because there are people who look for stuff not produced by the major publishers but who aren't looking just for self-published books. Just like indie music and indie gaming includes a wide range of companies and not just people "self publishing," sites that have a more inclusive definition of "indie" books attract a specific type of reader.
