If social investors are looking for returns on their investments, they need to see some numbers. Some things, like health programs, are more easily measured than other things, like environmental advocacy, but the burden of proof is on the organization to show what they are doing makes a difference.
Unfortunately, most organizations don't have the capacity to effectively measure their outcomes. Measuring takes time and resources and there isn't one all-encompassing measurement tool to determine an organization's impact--you can't do a randomized controlled trial on increased awareness about factory farming. You can support some of the few organizations that have the capacity to prove themselves, but if you aren't interested in the work they do, I would recommend supporting "high-preforming" organizations.
"High-performance" non-profits are up-and-coming, have good management and are attempting to evaluate. They differ from "High-Impact" organizations, the type that have made a difference and can prove it to you. As Sean says at Tactical Philanthropy, funding is needed at all stages of growth. High-impact is the "holy grail" for social investors, but like the holy grail, it's something to aspire to and not something we should expect. (Others would disagree, and for good reason.)
If you find your own holy grail out there (again, I'd recommend looking at this list to start), stick with it. But if you can't find a non-profit you are completely satisfied with that can give you good measurable results, don't despair. Look for one moving in that direction and support its capacity building. With your support and encouragement, you can help it develop the practices it needs to prove to you (and other funders) that what it's doing is making a difference.