Well, what’s your time worth? How much do you make an hour, when you break down your salary and work hours? Is that time best spent on social media marketing, or on other things? This isn’t meant to be a rhetorical question; perhaps it really isn’t worth the money. Some firms prefer to have fewer, higher-profit clients so that they can spend more personal time and deliver better service. That’s not a bad thing, but it might interfere with your budget and marketing goals. If you can hire a skilled social media person for less than half your hourly rate, you probably should.
If you’re the CMO, then you should be able to make the case to the CEO that it’s worth it to outsource this work. If you’re the webmaster, IT manager, or UI developer, it may not be possible to make the financial case right now, but you may want to make a more practical case that your job involves mission-critical systems that need proper maintenance and attention that you can’t focus on while you’re messing with Twitter posts.