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.
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.