Is it possible to get say, my snapchat account, from my IP address? I'm slightly worried because I wasn't using a proxy and this (presumably a) guy threatened to post my 'IP and snapchat' to a forum, but I never gave my snapchat. I'm not worried about my IP giving my location because I know the location(s) it gives are wrong, but I'm worried if they got my snapchat.
No, Your ISP can find your social media profiles by using your IP address and timestamp simply by searching into your browsing history in their logs.
So that attacker can't find anything about you with your IP address only. Maybe if he has more details like your name, he could use some services like pipl.com to search your social media profiles. That's a process called "Doxing" but that's not our topic here and it's not that effective without the right information!
No need to worry!
Yes and No.
IP (sic) address can be used to identify you. But in this scenario the treat is non-existent because all content is routed from your cell phone through snapchat's servers to the other person (aggressive guy.)
There are many situations where an IP address can be used to identify you, from internet usage, physical location and resources accessed. This is not one of them.
Your biggest attack vector would be the identification of other internet accounts by using the same username that is searchable via Google or other search engine.
The assocation of your username with your other Internet accounts may identify you physically such as instagram where you geotag pictures, facebook where you list work/location or other social media websites. The metadata collection here is most likely how he'll reference such an IP address threat.