I think that it's not possible at the moment. But I would expect that Meraki will implement it in the future. With "Smart Application Based Local Internet Breakout" the foundation is available in the MX. I could imagine that it will be expanded to internet-traffic also. But I would not wait for it.
If you really need it: Place a proxy server in a DMZ, configure it with two IPs and tell the proxy to use IP-A for general traffic and IP-B for social media. Then the Flow preferences of the MX can handle the rest.