I think there is an answer to this, and it has to do with identifying mid-level mechanisms and processes that recur in roughly similar ways in a range of different social settings. The social sciences can identify a fairly large number of these sorts of recurring mechanisms. For example --
- public goods problems
- political entrepreneurship
- principal-agent problems
- features of ethnic or religious group mobilization
- market mechanisms and failures
- rent-seeking behavior
- the social psychology associated with small groups
- the moral emotions of family and kinship
- the dynamics of a transport network
- the communications characteristics of medium-size social networks
- the psychology and circumstances of solidarity
Further, the social sciences can attempt to discover the circumstances at the level of individual agents that make these mechanisms robust across social settings. They can model the dynamics and features of aggregation that they possess. And they can attempt to discover the workings of such mechanisms in particular social and historical settings, and work towards explanations of particular features of these events based on their theories of the properties of the mechanisms. Finally, they can attempt to find rigorous ways of attempting to model the effects of aggregating multiple mechanisms in a particular setting.
What this comes down to is the view that the main theoretical and generalizing contribution that the social sciences can make is the discovery and analysis of a wise range of recurring social mechanisms grounded in features of human agency and common institutional and material settings. They can help to constitute a rich tool box for social explanation. And, in a weak and fallible way, they can lay the basis for some limited social generalizations -- for example, "In circumstances where a group of independent individuals make private decisions about their actions, the public goods shared by the group will be under-provided."
This approach affords a degree of explanatory capacity and generalization to the social sciences. What it does not underwrite is the ability to offer general, comprehensive theories about any complex kind of phenomenon -- cities, schools, revolutions. And it does not provide a foundation for confidence about large predictions about the future behavior of complex social wholes.