I work in online marketing and it's no secret that I blog, tweet and check in on Foursquare. Social media is largely responsible for some of my closest friendships. I've never felt unsafe participating and generally trust that the community will police itself and follow the unwritten rules. But sometimes I don't think about the consequences.

Then I had a bit of a wake-up call.

We had just been seated at a table at a local restaurant when the manager came by and asked if anyone was named Shea Sylvia. I nodded and she said I had a phone call. At first, I thought it was some sort of joke. A friend must have also been there or maybe Aaron was surprising me with something.

But when I answered the phone, I didn't recognise the voice on the other end.

"Hello?"

"Hey Shea. This is Brian."

"Who?"

I racked my brain, trying to figure out who I knew called Brian.

"I saw that you checked in there on Foursquare, Shea," he said.

My heart started to race. I've read articles about burglaries and stalking thanks to geo-location apps, but because I only occasionally link my check-ins with tweets, I thought I was being responsible. Standing at the front desk of a restaurant on the phone with a complete stranger was the absolute last thing I expected from a harmless tweet about meeting friends from the internet and a link to my location.

"I like to hang out with people from the internet too. Maybe we should hang out sometime. What do you think about that?" Brian asked.

"OK . . ." I said unconvincingly.

"Maybe we could ride bikes together."

"OK . . ."

And then he said, "Is this getting creepy?"

"Yeah, this is a little creepy," I replied, looking around the restaurant, hoping I'd spot someone on their phone. No luck.

"You probably shouldn't be telling people where you are on Foursquare, should you, Shea?" he said.

I was trying not to cry at this point. And then I hung up.

After I drove home, I called the restaurant and spoke to the manager. I wanted to know how she knew to come to our table to ask for me. She said the guy on the phone described me as having short hair and glasses and I was the only person who fit the bill. I'm wearing my glasses in my Twitter and Foursquare profile pictures, but I don't usually wear them outside of work. I don't think this was someone who knew me well.

I haven't been able to stop thinking about what happened. I'm angry. I feel like someone violated an understanding that all of us generally nice people online have – you don't cross the line. I'm also terrified. Who is this person? Who would do something like that?

I don't want "Brian" to scare me away from participating in the online communities that I love being a part of. But I will be more careful.

This is an extract from Shea Sylvia's blog