Four years ago, Rob Fishman and Darren Lachtman launched Niche, a company that matched social media stars with brands that wanted to pay them to promote their products. That worked well: Two years later, Twitter bought the company for a pile of money. Now they are getting the band back together. Lachtman, who left Twitter this summer, is rejoining Fishman, who has already launched Brat, a video startup. You can call them both co-founders, if you like.