I've never been entirely sure what exactly 'unpublished' constitutes. When I first started writing (mainly short stories) I posted all my work on various writing websites like www.deviantart.com and www.writerscafe.org.
When I started submitting for publication, I had some publishers who didn't mind me posting to these communities, some who flat-out rejected me because they no longer considered my work unpublished, and some who just wanted them set to 'only viewable to other members' so that it was only posted to a community and not the public. So, at this point I'm rather confused what the industry stance is on this matter. I do like posting to these websites for feedback, but now that I've started writing more seriously do I have to have to worry about them jeopardising my first electronic rights?
Let's start with the fact that there is no such thing as "first electronic rights"
First SERIAL rights describe excerpts you publish before the book is published, be that electronically or in print.
Second serial rights are excerpts you publish after the book is published.
If you elect to publish excerpts from your novel before it's sold you've licensed first serial rights. Format doesn't matter.
It's entirely possible an author could publish pieces of a novel pre-pubication to several outlets. (A WIP from Katherine Dunne, or Fran Liebowitz would certainly generate that kind of interest.) That's still first serial rights.
When you say "publishers didn't mind me posting to these communities" or "flat out rejected me" it sounds like you're talking about digital only publishers. Their contracts are for (territory) (languge) and (format).
Thus if you license World English for digital publication to them, they have those rights. First and second don't come in to play here at all. If they fail to publish, or publish and then revert rights to you after a period of time, you can re-sell World English for digital publication again. No "second" needed.
As to whether they want you to publish excerpts or not publish excerpts, it's their call. They run their business the way they see fit.
There is no industry standard on this because every publisher has different standards.
The trick here is to keep VERY detailed records of what you've published or posted and where. Print out the terms you've agreed to (ie don't rely on being able to view them on the site) and make sure you understand the terms.