This is kind of a strange comparison, but depending on your business advertising initiatives, you may consider either option. Here is the disadvantages to Social Media sites including Facebook vs putting a magnet, window decal or painting your car with a mobile advertisement.
Ads on automobiles - things to consider:
(1) Effective visual appearance and targeted branding for your market area (where you drive - you will have visibility)
(2) Your presence is as effective as your driving - people will not only judge you by your car ad, but the quality of your driving
(3) Does your target customer have the ability to write down your phone number or website address while driving?
(4) If you have a fish on your bumper or any other bumper sticker - will this distract or attract your ideal customer?
(5) Are you targeting customers that don't fit your model or services? Anyone & everyone can have the capacity to see your ad - but is your service offering tailored for any person or a specific market sector?
Ads through Social Media - such as Facebook:
(1) The page you develop is easily marketable and can reach to those within and outside of your market area
(2) You can tailor postings, visibility and networks to those you choose to market to
(3) You can invest in pay per click ads by specific topic or keyword searches
(4) The "Like" button is easy to select & your followers can have visibility to your promotions and contact information real time
(5) Your page can be "Suggested" to others without passing out your personal business card or flyer
Both initiatives may be appealing to your business, if you happen to select both, be sure to list your website for quick visibility to your social media sites.