1 you have complete and unrestricted access whenever you want - no ifs buts or maybes
2 at 12 too young for SM yet, but when you do allow it full access to that too - educate yourself on the methods youngsters use to evade such rules, new apps that parents and other adults aren't yet aware of, having more than one account, possibly in false names - very easy to do and very common. They use the one with the correct name as "safe for parents/relatives" and another under a slightly different spelling or nickname for stuff they know you wouldn't approve of.
3 no usage after bed time
4 if school work deteriorates due to having that's grounds for losing it altogether.
5 ditto if she uses it to belittle or bully others. Very often on mn there's much discussion of how to help children BEING bullied or belittled by other kids via phone but it stands to reason that mners kids are no less likely to be the ones doing the bullying. Sometimes it's deliberate but it's also possible to be egged on by others into being "braver" than they would be in real life.
A thorough discussion of acceptable and legal use before she gets it is a good idea.
6 be aware of possible physical effects - eg worsening eyesight, sleep issues etc