Ben Golub, the long-time CEO of Plaxo who oversaw the social networker’s sale to Comcast (NSDQ: CMCSA) two years ago, is leaving the company. Golub joined Plaxo in 2005 and says in a blog post that his “passion is in building new companies” and that he’s therefore leaving “to start working on my next company,” although he does not describe it. He is being replaced by Justin Miller, the former GM of Plaxo.com. Plaxo’s service lets users manage their online address books and also see what their contacts are doing on the web. Under Comcast, Plaxo has also been working to bring “social capabilities” to Comcast services.