Uhm. Depending on what context it was found in and what it was found with, that may mean absolutely nothing. Social security number is something of a lost cause as an identifier except for the purpose it was originally intended for, tracking your social security taxes so you can eventually claim your benefits.
However, as discussed in many of the scam questions, you can find out a bit by looking at the domain owner record. This says that since 1995, PBINFO.COM has been owned by
PENSION BENEFIT INFORMATION, LLC
333 S 7TH ST STE 2400
Minneapolis MN 55402-2411
+1.612-371-3008
[email protected]
That sounds like someone who might legitimately have your SSN, and if you find them at that address/phone/email I would tend to assume that they aren't a darkweb organization.
If the alert is trying to say that it thinks this is where the darkweb folks stole the info from, and you've quoted it accurately, it's not exactly clearly written. So I'd look Really Carefully to see if the note actually came from Discover, or from someone pretending to be Discover to get you to give them additional info "to help us resolve this."