I'm not even sure if I've ever went to a bar alone in my whole life, but from my observations it's not very common to talk to strangers at all here in Germany, I guess the best chances are outside in a beer garden and not in a bar or cafe, or if the venue is packed enough that you get someone seated at your table (or vice versa). Then again, if you're out with people you're probably more focused on your group so you don't necessarily notice the strangers connecting, but I still doubt I'd missed it all the time.