Billionaire Elon Musk said in a tweet on Monday that Apple Inc has mostly stopped advertising on Twitter.  Apple and Twitter did not immediately respond to requests for comment.  Earlier in November, Musk said Twitter had seen a "massive" drop in revenue and blamed activist groups for pressuring advertisers.  Several companies including General Mills Inc and luxury automaker Audi of America have paused advertising on Twitter since Musk completed his purchase, while General Motors Co said it had temporarily halted paid advertising on the social media platform.  Apple spent an estimated $131,600 on Twitter ads between Nov. 10 and Nov. 16, down from $220,800 between Oct. 16 and Oct. 22, the week before Musk closed the Twitter acquisition, according to ad measurement firm Pathmatics.