Wall Street Journal: Twitter’s biggest dilemma is that it can’t figure out a way to get more people to use its service. The person in charge of that critical mission is yet again being replaced, casting more doubt on Twitter’s ability to solve this riddle. CEO Jack Dorsey said technology chief Adam Messinger will be taking over the core Twitter product from Kevin Weil, who is leaving the company. A Twitter spokesman said the company isn’t looking for a new product head nor is it planning to.