Ha Ha autumnlights - very eloquent.
To be honest I have had to work to not be subsumed into my children. Having had 18 years at home with 4 of them, either full time or studying part-time, there have been points where my identity has been lost for a while. I think it's really important if you are going to be a SAHP for long periods to make sure you have other interests/motivations just for your own mental health. I think most women do manage a balance but sometimes SN and illness in the family can make striking a balance impossible and this is often where mothers do lose out more than fathers.
The FB photo issue could possibly be seen as a symptom of the subsumed woman but I don't really think it's anything as deep as that. Most parents have a million great photos of their kids to every one decent photo of themselves - it's probably more likely vanity or laziness, (well it would be with me anyway.)
I think anyone who feels that their only role in life is being a parent is selling themselves short but then so are those who define themselves solely by their career. I think we are all more than this.