My mum's on Facebook. She constantly leaves messages for me on the social networking site, reminding me to put my bins out, or water my houseplants – even though I'm 27 and have lived away from home, unscathed, for 10 years. She also does it to my little sister, who suffers a constant stream of web-nagging.

My dad's on Facebook too, but he doesn't leave me messages. I can't see his profile, just a blurry photo and the stern injunction: "You must be friends with this person to see his full profile." This is because we are not friends – either online or in real life. We have never even met.

My father was gone from my mother's life before I was old enough to be aware of his existence and, as far as I know, has never attempted to get in touch with me or offer any support in my upbringing. This isn't particularly unusual, certainly in my generation. I consider my mum's long-term partner, who came into my life when I was 13, to be my father in every practical sense of the word. Although I was an angry teenager I have never felt particularly anguished about my biological father's lack of interest. If divorced-child trauma were measured by a scale, like earthquakes, mine would barely register – just a tremor really. I've got lots of family on my mum's side – plenty for anyone.

But a few months ago, as I logged on to Facebook, my eye was caught by a familiar surname in the "People You Might Know" application. This little box points out to the user people with whom you have friends in common. It was my father's surname that I glimpsed, which is reasonably unusual, and the first name Lee-Anne. The thumbnail-sized image was not of a person but a grainy black-and-white pattern. Without even thinking I clicked on her name, and because Lee-Anne left her profile public, so anyone can see it, I was taken straight to her page.

My eyes slid down the page. She's younger than me. She lives in the same town. She's pregnant. And in the space of two or three minutes I realised that she is my half-sister.

Facebook has another helpful feature – an add-on application that lets people display a family tree on their profile page. She had filled in her father's name. It's the same name as my biological father. There was another thumbnail picture, which I couldn't really make out because it was tiny. There were more photos, one captioned "Dad", which I clicked to enlarge. I was breathing shallowly.

I have my mum's hair, her mannerisms and her ways of speaking. We look a lot alike. Everyone says so. But here was a man with the same eyes, eyebrows, cheeks, even ears, as mine. This was my father, the man I had never known, and there he was staring back at me. The shock of it hit me suddenly, and I felt tears running down my cheeks as I read the comments he had left on Lee-Anne's wall – how excited he was about his impending first grandchild, how he hoped to make it back to Wales to see her very soon. I was so overcome that I switched off the computer and retreated to bed, where I lay awake with my mind in overdrive.

Two months later, I have turned into an online stalker. That first accidental glimpse has turned into a low-level obsession. I check Lee-Anne's Facebook page every day, reading her complaints about the discomforts of late pregnancy. I have followed a link to a video blog that my father updates regularly with clips of himself doing karaoke and telling jokes. I now know where he lives, works and drinks, all information I never had before and all just from looking at web pages. I haven't got in touch, but I can't seem to do the sensible thing and consign the web bookmarks to the recycle bin.

I am fascinated by these people even though I know that what I'm doing is borderline odd. I don't want to meet a man who is only biologically my father, but I can't stop watching him either. I like the fact that I know more about him than he knows about me. I can satisfy my curiosity, without the fear that he will reject me. Nothing has actually changed in my life – and yet everything is different somehow.

And this week, Lee-Anne had her baby. I'm an auntie. She has already uploaded hundreds of photos to her Facebook page and I have been sitting at my computer, watching this brand-new person's life unfold over the first few days, feeling an odd combination of excitement and horror. I hope Lee-Anne's boyfriend will stick around to raise this child. Out of sight, out of mind is not an option for absent fathers anymore, at least not for mine. I am watching him now, and I can't seem to stop.

Names in this article have been changed.

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