Never a social butterfly at the best of times, Snape had been in a particularly bitter mood ever since George's patronus had summoned him to Diagon Alley, where Riddle had staged another of his asinine displays of egomania, using for mere set decoration the body of one of Snape's oldest crushes/foes/friends.
His thin mouth twisted sourly as he mused. Ironic, to end like that, after somehow surviving so very much.
As he brooded, he kept Regulus company, the two Slytherins sharing the same corner for most of the meeting. When Regulus went to talk to his brother, Snape stayed behind. There was no-one here he wanted to talk to. Instead, he started composing a letter of condolences to Narcissa.
He knew perfectly well that it would be one of the few letters that wasn't purely for form's sake: an attempt to curry political favour with the Minister.