I’m building a website that will let users put markers on a map and post comments related to the markers.
Do I need to release the markers data and the comments as CC or PD?
Some users will want their comments to be private or to be seen only by their friends.
You’d better ask this in the legal-talk mailinglist.
But on a basis of IANAL: if you do not merge both databases, or just use the openstreetmap tiles and display the markers on top of the tiles as a regular mashup then that data does not need to be released under CC-by-SA.