Almost all frum places allow mixed seating even in Williamsburg. Its mixed dancing they will not allow
What people do is have areas of mixed seating for those who wish it and seperate for those who dont. (The host should know in advance who wants what)
As far as the “socialize” comment. I was at a chassidic wedding in New Square (There is some big hall there) and while I did not stay for the meal, I was there for the Smogosgord and it was a mixed Smogosboard.
And what really frum people do who go to a mixed seating event is sit next to their spouces on one side and a member of the same gender on the other.