For example, you might get a write up in a blog that then gets you a lot of karma on social bookmarking sites (Digg, del.icio.us, etc.) and then that gives you the immense traffic that makes you succeed. Do you mark that as being based on the blog write up, on being based on social bookmarking? (which isn't even on your list)
I would call social bookmarking part of on-line word of mouth.
And in the situation you describe, I would ascribe the major cause to the blog write-up because you can trace it back to that. There are certainly many situations where you can't trace it, i.e. it is a murky combination of many things. In those cases, I think the correct choice would be more than one above.
That's interesting, because I consider "word of mouth" to mean verbal communication (such as telling people on Twitter, IMing, e-mailing friends, etc) rather than voting up a link on Digg or del.icio.us, say.
Perhaps I should have separated them, then (assuming other people think like you do). I guess I consider them the same because people are similarly taking the initiative to share the link on-line with a community, be it their friends or a more general community.
It's not the general intent ("sharing") or the community that differentiates them to me. It's the action. Adding a bookmark to del.icio.us is quite different to saying "Wow! Check out this awesome site http://example.com/" on Twitter to my followers - not at a technical level, for sure, but in terms of potential effect.
For example, you might get a write up in a blog that then gets you a lot of karma on social bookmarking sites (Digg, del.icio.us, etc.) and then that gives you the immense traffic that makes you succeed. Do you mark that as being based on the blog write up, on being based on social bookmarking? (which isn't even on your list)