I could imagine this is a problem as a company grows. I've run a ton of Facebook ads every day for three years, so I know the ins and outs of how to do a performance-based ad campaign. Compare me to someone who is good or dabbled in SEO, display, Facebook ads, Adwords, etc. and I'm probably the best choice to run your Facebook ad campaigns. So you could be replaced by a specialist. I think that's why you should look at it less as a curse and more as the way to know what path you want to take (VP of Marketing, go to another startup, grow into a specialty at your current company, or take a more managerial role in marketing at your current company).