Bitkom polled 1,013 internet users over the age of 14 years, including 703 active users of social networks, and found that fully 85% consciously refrained from posting information that disclosed certain personal details.
And 43% indicated their participation in networks was not under their real name.
"Most social network users are very careful with personal information and opinions," said Susanne Dehmel, managing director/trust and security, Bitkom.
"They decide what content they will share with other members of the network depending on the situation and content involved."
According to the survey, 41% declined to make any comments on religious content while 37% avoided expressing a view on political issues.
Family privacy was another issue for a significant proportion of respondents: 39% would not post pictures of their children.
Brands' efforts to engage consumers via this platform also run into a wall as one third said they did not comment on products or services.
Almost two thirds were ready, however, to give up information about their sexual orientation, and 45% to add information to photos in which they saw themselves.
A separate Bitkom survey of social media use and television revealed that more than a quarter of social media users (27%) also used sites like Facebook or Twitter, or communities like Couchfunk, to follow TV-related content.
Most do so passively, simply reading what others have written, but almost one in ten (9%) actively participate in discussions.
"The exchange with the community gives the viewer the feeling of sitting together with others in a room and watching TV," observed Timm Lutter of Bitkom, adding that some programmes aired opinions and survey results from social media.
Data sourced from Bitkom; additional content by Warc staff