Hmmm, I would go for ResarchGate - you're right, technically, the rules only cover personal and institutional repositories, and ResearchGate is a third party private host. So yes, if you want to stay strictly in the rules, its not allowed.
But in practice, everyone ignores that. The big publishing houses know they're on borrowed time if they mess with that (there is already a huge backlash against them in academic circles). Plus, in practice, there's no means for them to check the version you posted against the original one you submitted, so in other words, they cannot easily detect exactly which version you posted (unless its the final formatted one).
Technically speaking I don't think the rules would allow you to post your piece on WordPress for the same reason - its a private, third party host, not your own or your Instutition's website (You could link to it from elsewhere though). The reason ResearchGate is good is because its well trawled by Google Scholar bots. So the link to your fulltext paper is more likely to pop up if anyone does a google scholar search.