1- Maybee they sell your personnal photos
The most of photo sharing like Instagram has the right to use, edit, delete or share your photos. and we find also Twitpic who has the rights to share your photos with 3rd part , in 2011 Twitpic signed a contract with World Entertainment Celebrity News Network competent in celebrity news . What i want to say is for example if you take a photo with populaire personee they can share the photo or sell it with your own permission and they can delete your name from the photo , do you know why ? , because you accept that in terms of use.
2- Maybee you can't delete your account
If you're one of Skype users maybee you know that you can't delete your account , skype is one of programs that don't allow to users to delete their accounts , and if you read the help page of skype they suggest to delete your informations but you don't know if they will keep it on their servers or not .
and Wordpress is another example of websites that don't allow it users to delete their accounts.
3- They can follow you online
in facebook's terms of use you agree to allow facebook to follow you using cookies when you leave the website , when you sign in to facebook they will track you in every website contain like or share button and this relate them with your acconut and email for 90 days. and it's the same thing for many other websites.
4- They can give your informations for legal entities without getting your permission
The most of entreprise's terms of use don't tell it users in what they will use their informations and if they will share it with 3rd part or not and if they will ask you to share your informations with legal parts or not.
5- They can stop you from raising any lawsuit against the company
After a serie of hacking security against sony which cause disclosure of millions of users informations, many peoples raise a lawsuit against Sony . and after some months sony update it terms of use and stop any one from raising a lawsuit for any raison. and the same strategie you can find it on Microsoft and AT&T.
6- Even if you delete your informations they can keep it on their servers
Don't think that when you delete a photo from facebook you finish because if you read the terms of use of facebook you will find that they keep your photos on their servers for unkown period of time. And the same thing on Twitpic but the difference that your photos will stay forever on their server for law raisons. Twitter has clairful terms because your photos will stay for 5 weeks on their servers before they delete it. but make in mind that in these duration anyone can watch your photos if he knows the Link.