Awesome idea! The more of this sort of thing the admins see, the better!

we'll still be seeing those annoyingly godawful IMVU ads as well though regardless. :/

Umm... If FA requires the funding from those ads to run, then they can't simply remove them unless there's an alternative to replace them with. Like it or not, Adult ads probably bring in more cash than standard ones. Also, apparently they only show up on mature/adult submissions anyway? So... yeah.

A site also can't operate without users. You're missing the point. A petition itself doesn't actually do anything other than tell admins what users think or feel. A lot of users are against the explicit third party ads. There is no harm nor foul in simply telling the admins that. All it is is user data and feedback.
If you're fine with the ads, there's no need to sign a petition that says you don't like them. Easy.

Also the ads appear even if you ever even once clicked something mature and they also occur even if you're working in SFW mode which impacts a lot of minors which at the end of the day could get the site shut down.
So....Yeah.

You know what, in case any of what I said sounded like I'm vaguely in support of those new ads, then I hereby take it all back.

I saw some examples of those new ads yesterday, and they clearly originate from the deepest, filthiest pits of the interwebs spam hell, and must be sent back there, asap. X3