Friend of an ex added me on Facebook?

I suppose "frenemy" might b b better term, but yesterday ou of the blue an old friend of my ex's added me on facebook. These guys have known each other f quit literally the last two decades. And while I do know the friend a little bit (I can count our conversations on one hand) I don't know him that well. He actually went ou of his way to search for me on facebook (due to my security settings) and he waited until now to add me when he could have done it at any point in the two years my ex and I dated, not when we're no contact. What's going on here?
Friend of an ex added me on Facebook?
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