If your end goal is to drive direct-from-site sales/leads/website-based engagements, then lead off with SEM.
If your end goal is to drive brand or product awareness or re-engage with prior users/visitors, then lead off with paid social.
Remarketing clean up: Someone searches and enters the site on a Google ad but abandons – that is when paid social (or really any programmatic) can hunt down the searcher with the same thirst for blood as the TSA agent at LaGuardia airport, searching for my half-full water bottle.
Capturing the mid to low funnel: A person busy arguing political points on Facebook who is shown an ad pushing mouth-watering bacon will then go do even more research on Bing (I refuse to call it Microsoft) in hopes of finding a better discount code.