Question About iCloud Email Uses

Hello, I need help understanding the usage of iCloud Email. I find it interesting and cool how I can pretty much access any email I get right from iCloud.com whenever I'm not on my Mac or iPhone. Basically as having most of the iCloud features as in my iPhone and Mac.

But one thing that has me wondering is what can I use iCloud email for? Like, basically I know that I can email with it. But what I'm wondering is if I can use it with anything. Things that would be used such as Facebook login, of Twitter, or blogs, or anything that any other email such as @yahoo.com, @hotmail.com, etc. can be used with. I'm wondering this because I'm interested that I can replace whatever I'm using such as Facebook and others with an iCloud email.

Another question too is if I can use my iCloud email as my Apple ID. This would be cool to use with if I can focus it all on iCloud email. I know that I can sign into iCloud.com with my iCloud email as well, but I I want my iCloud email to be a sole stand-alone email usage. Not relying on another email sevice as a main account to it. (If this isn't possible, then it's ok, it's not a main concern. A good thing to think about it is that the other email can act as a rescue, just in case)

To sum it up, I'm wondering if iCloud email is available to use with any other website sign-up, and if my iCloud email can be used as my Apple ID. Thanks, and I hope someone answers this!