Thanks for asking. Yes, its somewhat similar (IMO, as similar as facebook vs Twitter). Meetup is pretty close though - the core philosophy of meetup is you want to plan ahead and find people of similar interests. you need to have a continuous meeting groups. Its a big overhead and not a simple task to lets say conduct a garage sale or find a buddy for running around a park.
I would say tellStreet is more like twitter whereas, others are more like email groups.
I suggest you try meetup again, as they have similar list style feeds for global and groups you're subscribed to, exactly like twitter or your site. The only real difference I see is you map them.
I would say tellStreet is more like twitter whereas, others are more like email groups.