No, and yes
The No part
A right is not an obligation: having the right to free speech does not mean that you have to speak freely for example.
A contract (including ToS) grants rights and obligations on each party. In general these are complementary, one party's right is the other party's obligation: the citizen's right to not be unlawfully detained imposes an obligation on the state not to unlawfully detain its citizens, for example.
Enforcement of a contract (either through internal processes like moderation of content, or externally through courts and tribunals) implies that there is a breach of an obligation by one party and a consequent infringement of a right belonging to the other. The choice to seek enforcement is entirely at the discretion of the person whose right was violated - if they choose not to enforce their right then this is a waiver.
The No part
Waivers happen all the time because contracts are breached in immaterial or inconsequential ways and people just move on.
Many contracts contain a clause to the effect that a waiver does not amount to a permanent waiver and that they reserve their rights on future breaches. This is important because a contract is more than the written agreement; it involves the entire conduct of the parties. Without such a clause in say a lease contract, a lessor who has waived their right to terminate the lease for late payment of the rent in the past may be barred from executing that right in the future.
Similarly, where a company has the same contract with many parties: their behaviour in dealing with the same breach by Party A, B, C & D may be relevant in how they deal with that breach by Party E. If they are not consistent, and the inconsistency is arbitrary, malicious or unlawfully discriminatory, then they can be in trouble. However, if the inconsistence has a technical explanation (for example, posts by new users often flagged for moderation while experienced users aren't) or is a reasonable use of discretion then that is OK.