The answer will depend on what you have on your profile, and what you tweet or post. It is important that when using SNS you are in control of who can see your account details and content including photos and albums, posts, status updates and any personal information. On Twitter, you can set your account to private by selecting 'Protect my tweets' so you can then accept (or decline) requests to follow you. In the case of Facebook, choosing a basic 'Friends' setting for every option would initially achieve this. However, you are able to customise each option further, and can limit the information that certain individuals see. It is a good idea to use the "view as" option, to check and see how your profile appears to strangers, and that the information you want to remain private or 'friends only' is not visible. If you are unsure about how to use the settings available, treat all information that you post as being public and act accordingly.
Think carefully about whom you are friends with, and which friends can access what information. It is a good idea to remove any "friends" or customise the privacy settings for current friends, if access to your personal activity could compromise your position, for example parents with children at your school. However, whatever setting you use, it's important always to think before you post because 'Friends' settings do not guarantee privacy. Sharing content with others could mean that you lose control of it, if friends pass on your information, for example.
Think carefully about comments you post on friends' walls, if their profile is not set to private your posts will be visible to anyone.