Almost as soon as I echo the question “who are you writing for” then a prime example appears in the pages of the Observer Woman magazine.  Rachel Cooke calls those obsessed with motherhood “boring, selfish, smug” and cites this piece of evidence:

Let me give you an example. The other morning, while I was thinking about writing this piece, I logged on to one of the dozens of websites now devoted to all things baby-related. The discussion subject of the day – email us! – was the funny ways kids mispronounce words. Really. To which I say: new mothers, by all means, tell your own parents, or a close friend, about how your son said the word “bottle” and made it sound like “bottom”. But don’t be incontinent. Don’t tell the entire world. Telling the entire world will make people, and not without reason, think that you have lost your mind.

Cooke’s false lemma here is to equate putting something on the internet, with, OMG, TELLING THE WORLD!  Confessing something in an Observer column could be described in that way.  Participating in an online discussion, while (technically) available for everyone to see, is something quite different.  It is participating in a community, based around a shared interest.  If the sites are appropriately labelled (and if they are called Alpha Mummy, Mumsnet, Babble, and MumsRock, then I would suggest that is the case) then the real mystery is why anyone without a child would go anywhere near them.  They are narrowcasting, not broadcasting, and joining a discussion there does not equate to “telling the world” or the self-centredness that Cooke implies.
The Internet, with its millions of users and trillions of pages, carries fan and hobby sites for absolutely any human activity one can imagine, including knitting, Buffy the Vampire Slayer, and foot fetishists.  They might appear obsessed, but then any highly specialised conversation seems that way to an outsider.  Since we only come into contact with these people when we see them online, we do not credit them with any other interests, other than what we read of their online thoughts.  So it is with the parenting sites mentioned above:  They are populated by a self-selected group of people, with a narrow remit to talk about one subject.  They are not representative of a wider trend, any more than the so-called Dummy Mummies Cooke has apparently encountered at London social events.
Plenty of my friends now seem to use Facebook as a place to post pictures of their babies.  Does that mean that they are obsessed?  No more so than all my childless mates, who only ever post images from their holidays, or of weddings.  We choose to only record certain things, tiny slivers of our lives.  Thank God they don’t post pictures of themselves on the train, or worse, on the bog.