The two most important federal data sources for volunteering are the Volunteer Supplement to the Current Population Survey (CPS), and the American Time Use Survey (ATUS). Each has its advantages for particular purposes.
The most important difference between them is that the CPS asks respondents whether and how much they volunteered over the previous year, while ATUS asks what people did the previous 24 hours. As a result, only the CPS can answer what portion of the population volunteers over the course of a year, and only ATUS can answer what portion of the population volunteers on an average day.
To estimate the total amount of volunteering that takes place in a year, ATUS is much to be preferred to the CPS for a variety of reasons. First, respondents are much more likely to remember what they did yesterday than every day for the previous year. Second, the CPS volunteer questions are always asked in September, while ATUS surveys people throughout the year, better capturing any seasonal variation. ATUS includes specific followup questions to determine if activities reported were volunteering, and incorporates additional time associated with volunteering, such as travel to volunteer, and waiting to volunteer that a CPS respondent would likely forget about. Also ATUS respondents report only for themselves, while CPS respondents give data for all adults aged 16 and over in the household. Finally, the CPS publishes median rather than average volunteer hours, a statistic that cannot be scaled up to the full population.
Both sources provide data on the demographics of volunteers, but those from the CPS are more readily accessible. CPS has information about the main type of organization volunteered for, the activities performed for the organization, and how volunteers became involved. ATUS provides detailed data about the nature of the activities volunteers performed.
Both sources are valuable, but you may need to use one or the other for specific purposes.
ATUS home page: http://www.bls.gov/tus/
For the current volunteer supplement release: http://www.bls.gov/news.release/volun.nr0.htm
(earlier releases available elsewhere on the site)