When my wife and I first started dating, we had met through a dating site.
There were a few other woman I had casually dated, and one of them was someone I was still seeing as well as the woman I would eventually marry. I had reached a point where I realized I didn't see anything happening long term with the first woman I was dating, and I wanted to date my wife exclusively.
My wife (then GF) was seeing another guy as well. At the same time I decided it would be best not to see my other lady friend, my GF decided to just date me exclusively. The other guy she had been seeing was not too happy about it.
The other lady I had been dating had a friend that she called a couple of times a year just to catch up with and see how he was doing. She had dated him briefly and they ended up casual friends. She called him to see how he was doing he said not too well, the woman he had been dating just dumped him for some other guy...... yup, her friend was the same guy my GF had been dating.
They shared information about us, she told him about me, he told her about my GF, then we (My GF and I) both got phone calls.
The other lady I had been dating called me to let me know all the things my GF had told her other guy friend about me.
My GF got a phone call from the other guy to inform her that I had been dating another woman at the same time I was dating her.
They didn't realize they weren't telling us anything we didn't already know so it didn't do any good.
But considering there are thousands of people on the different dating sites, I thought it was a real coincidence that the other people my gf and I had been dating knew each other, and she (the other lady I had been seeing) decided to call him right when all of this was happening.