Very often I find that informal and social learning are grouped together. Sure there are some overlaps between the two, but they are not necessarily one and the same. Let's start by looking at their definitions.
Social Learning is participating with others to make sense of new ideas.
Marcia Conner, author of The New Social Learning (Ref)
Social Learning is a natural continuous process of learning with and from others.
Jane Hart, author of Social Learning Handbook (Ref)
Informal learning is the unofficial, unscheduled, impromptu way most of us learn to do our jobs. Informal learning is like riding a bicycle: the rider chooses the destination and the route. The cyclist can take a detour at a moment’s notice to admire the scenery or help a fellow rider. This is in contrast to formal learning, which is like riding a bus: the driver decides where the bus is going; the passengers are along for the ride. People new to the territory often ride the bus before hopping on the bike. Jay Cross, author of Informal Learning (Ref)
So while Social and Informal can sound like the same thing, they actually form two different aspects of learning.
As you can see social learning is not always informal and informal learning is not always social. As Jane Hart terms it, the formal + social learning can be referred to as 'social training' while informal + social learning can be referred to as 'social collaboration'.