Buy what you know is certainly a good heuristic. If I were a partner at a fund that invests in Internet-enabled networks of engaged users, I would be very alarm

Buy what you know is certainly a good heuristic. If I were a partner at a fund that invests in Internet-enabled networks of engaged users, I would be very alarmed if companies like AirBnB and Etsy started to drift outside the scope of "what I know". Also, what I know probably shouldn't be so limiting as what I use. For example, one of Warren Buffet's best investments was Blue Chip Stamp -- a discount coupon book publisher. I doubt Warren Buffet ever licked a discount stamp (other than as a stunt). But he does understand businesses that generate float. He is the master of investing float.