This program is the easiest-to-use method for accessing and mapping decennial Census and American Community Survey (ACS) data. Unlike American Factfinder, it contains both historical and current data in one place. Currently Social Explorer has less 2010 data than American Factfinder (AFF has the 2010SF1) .
For ACS data, to access Margins of Error (MOE) in Social Explorer, make sure you are using the Census Bureau's tables. Social Explorer defaults to using their tables, which are cleaned up, easier to use versions of census data. These are appropriate for some uses, but researchers needing more authoritative data and especially those needing Margins of Error will have to switch to the ACS tables, in the tables portion of the process of selecting data in "Reports".