Yes, because they are literally the only person on the internet who goes jogging. Jogging you say? Must be Jane Smith of 15 Acacia Avenue, Surbiton. Tennis? That's Linda Sidebottom from Neath.
It's the cross-referencing of details. It wasn't this 
, but think "I started this morning still on a high after winning my tennis club's monthly tournament last night, and then I found cat shit on my fucking doorstep. Well, I say found. I stepped in it. In my best work shoes", from a fairly prolific poster whose area I already know.
As it happens, I know Greg [not real name] who lives in the same town, who happens to have been nattering about said tennis tournament on FB. So now, I just need to ask him the name of the woman who won, and I now know MNer-with-cat-shit-on-doorstep's real name.
If I was of a stalkery nature, I would know where to go to make sure I ran into her in RL one evening, and I could probably get her address easily enough. If I was a gossipy, malicious nature, I could then show her posts in, say, the Sex topic to Greg.
This is a lightly fictionalised account- I really did recognise someone like this, but I didn't take any further steps, because I'm not that kind of fucker. But I could have done, rather than thinking,"oh, you must be at Greg's tennis club!"
So this is why people say "my hobby".