I strongly believe that a person's social media is theirs to curate, and no-one owes anyone a 'friending'.
That said, it sounds like your MIL is not interested in having a relationship with you, or in you as a person, at all.
She didn't make an effort to visit or get to know you prior to children, she publicly and passive-aggressively expressed her disapproval of your marriage to her son on your wedding day - if you find that funny, more power to you - and she has rejected your friend requests and/or deleted you multiple times now.
Passive-aggressive is clearly her language:
Maybe she's having a hissy fit at your not indulging her matriarch fantasy by making her the very first to know way before everyone else, and now she wants you and your husband to chase her and coddle her feelings.
Maybe she doesn't approve of your having a second child, now that she understands the level of access she'll have and that she won't be the ultimate mother in charge.
Then again, maybe she's just a temperamental wench.
Or hates the memes you post. :)
Regardless, you are right to discontinue rewarding her nonsense by chasing her or having your husband do so. Focus instead on people that support and respect you.