Adam,
The way I'd do that is to simply create a Website/URL field in JomSocial for Facebook and LinkedIn profile URLs. Then, have JFBConnect and JLinked import the user's Profile URL into that field.
The benefit of that is that it will work for any user.. so your Facebook/LinkedIn users will have it automatically filled out, but users that create an account through JomSocial will be able to add it as well if they want.
I hope that helps explain. if you'd rather something automatic, it's possible, but takes more work and would only be for your JFBConnect/JLinked users.
Thanks,
Alex