The one and only time I took a dog to a leashless park, I walked in to the fenced area with her, took off her leash, and had a large chocolate lab walk up and pee all over her (she was 2 yrs old and had been spayed at 6 months - so no signals coming off her). The owner stood there laughing and did nothing to correct his dog. I put her leash back on and walked out. Never attempted to socialize with unknown dogs again.
The only dogs mine hang out with are dogs who I have met before and seen how their owner's handle them.