I think its also a question what about your forum is.
I am running a gaming forum and there are thousands of other boards doing so.
Many of them have big companies in their back with a lot of staff.
I will never reach their member count but I'll try
Point out what makes your forum special and advertise with this.
I know its damn hard but you will make it.
Getting the first thousand users is hard. The next thousand is easier, believe me