Some have a specific purpose. Look for people in your field and use the sites they are using.
'Create' as many profiles as you have time to keep up to date.
Consider what information you should disclose. Make sure you only publish information on your research profiles that you are happy and legally allowed to disclose to others.
Before you create a new profile consider whether you have enough time to monitor the profile and keep it up-to-date.
Check copyright conditions and publishing agreements. Make sure you have the right to share materials.