While what R0J0hound said is true, it also helps to use a little common sense when identifying spam.
Is the linked site suspicious? Does the link contain any referral or affiliate I.D.? Is it off-topic from the conversation?
If you can say yes to any of those questions, then it's probably spam.
I did, however, do a quick image search of his avatar and he does indeed seem to be a user of that site.
It doesn't necessarily mean anything bad though. It just means that he's familiar with it and thought to suggest it.
But, do whatever you want.