An excellent source of overall statistics about the nonprofit sector is Giving USA. This annual report by the Lilly Family School of Philanthropy at the University of Indiana, provides giving estimates by source of contribution--individuals, foundations, bequests and corporations--and how that money was used by nine subject categories, such as religion, health and education.
You might also be able to find the statistics you're looking for in Foundation Center's special reports on funding trends for specific topics, grantmaker types, U.S. regions, and internationally. This Knowledge Base article also lists Foundation Landscapes, free custom web portals that provide aggregate data on selected subjects. You can also order custom statistical searches from Foundation Center's Knowledge Services.
If you just want quick numbers for internal use, such as benchmarking, you can look at the aggregate numbers in our database, Foundation Directory Online (FDO). This tool contains data on 11 million grants awarded since 2003. It is searchable by criteria such as subject, population served and geographic location. FDO provides statistics such as the total dollar value of the grants that match your search criteria or the average size grant awarded by a funder.
But please be aware that FDO's statistics are aimed at helping nonprofits gain a better understand who is giving and receiving grants in their area; FDO is not meant to provide hard statistical data for analyses of the social sector. If you are an arts nonprofit, for example, FDO can help you find potential funders and collaborators. But if you are a scholar trying to determine how much arts funding was awarded in a given year, FDO is not an appropriate source.