It was unable to do so when I tried. How motivated should I be to give this any more of my attention?
I haven't heard of anybody else having this problem in this case (but also not of anybody else explicitly doing this part).
The only thing I can immediately think of is that you might have some general setting to do with Apps disabled. If you don't have any Apps on your account, then there's a good chance this is the problem, and I think you should be motivated to try temporarily turning on Platform by turning on Apps on the first setting on this page. Otherwise, I'd say it's your call. Thanks for trying, any which way.
Also, why does Stellar want access to my Facebook account?
I do not know. But when I recentishly turned on Platform to allow an App to integrate, I concluded (as someone who is inclined to be somewhat conservative about such things) that there was nothing to worry about if it was temporary and there was no immediate damage while the App was permitted. (If it does something like post embarrassing things to your timeline or someone else's, it would show up in your Facebook Activity Log. (Mine hasn't shown anything for when the Stellar app was integrated.)
Is there any danger of it spamming everyone on my friends list with ads for Stellar?
Nobody who reported back on Facebook saying they'd done it complained of anything untoward happening. It's conceivable that integrating the app even temporarily could mean that your friends see slightly more Stellar apps (if such things even exist) instead of the usual types of ads they see, if the ad algorithm took it into account or something? But I'm guessing that's not what you mean, that you meant more like Stellar making posts or sending messages via your account to friends, which doesn't seem to have happened to anybody who claimed the Stellar.
Thanks for your comment; I shall update the post with my advice about Platform in case it helps anybody else.
Most of the effects of allowing an app access are permanent insofar as most of the effects are them scraping your data, which they have no reason not to keep.
Edit: This post is obsoleted by this post; please see that one instead.
MIRI are currently holding a donation-matching challenge, until Friday 15th August. You can donate and track its progress by going to the Donations page.
Also, to quote the MIRI Facebook page:
Registering for Stellar requires just a username and password, no e-mail or verification required. To get the free Stellar, you have to have Facebook Platform turned on (see the first setting on this page) and allow the Stellar App to temporarily integrate with your account. Send 1000 STR to 'miri' or their address (gHhshpzDcfRsie2qxjjHqrsTRe3JSCaUeN), and you will get back the 1000 STR. Once you receive back the 1000 STR, you can then remove the Stellar app like any other in your Facebook settings.
Edit: The promotion seems to have been reduced. See these two comments.
Edit2: The promotion seems more-or-less gone now; consider the part of this post about Stellar irrelevant.
I did this and it took me maybe three minutes (including time to take notes). The Stellar website is self-explanatory. If you encounter difficulties at any point in the process, or if I've forgotten some part of the process, feel free to comment on this post to that effect.
To track follow-up on threads like this: If you donate because of this thread, please indicate that in this comment (you can do so anonymously there if you prefer), and feel free to state that you have donated as a reply to that comment.
(If you're thinking free cryptocurrency is too good to be true, and wondering what the founders get out of it: My understanding is the reason this works is that early investment in a cryptocurrency bolsters it, so that giving away some for free at the start is actually a smart thing to do at first for the founders. No gimmicks or tricks about it asking for card details or anything dodgy like that.)