Browse the group directory in each professional / networking site and join those relevant groups wherein you can interact with group members, share your knowledge, write your job interests, etc.., You may have several groups when you throw a search, open each group and read their content / group information which helps an individual to decide whether to join the group on not! Also, there are limitations in the number of groups an individual can join, so choose wisely.
You can find professional groups on Linkedin, Xing, Ning, Yahoo, Facebook, Google, etc.., Most of the groups are open to all individuals and there are certain groups which require Moderator’s approval. Join and spend some time everyday to follow the discussions on your group list, this could be a platform to gain knowledge and to explore yourself in the social media on to the professional front.
For example: Professionals who are looking for job in the market, search for the groups with relevant words like opportunity, job, posting, market, etc.., if your target is a specific country, search on the country name, if you intend to gain knowledge in specific skills, search on the technology name.