a.) Meet in a meeting room for a talk about some topic. b.) After the talk, anyone interested would walk over to a pub for some drinks and conversations.
We noticed roughly the same thing that you noticed - we'd lose roughly 90% of our attendees between step a and b. So, we decided to switch things up a little bit and started meeting in a pub. A local watering hole had a private room that they'd make available to us once a month. Then, we'd arrange for some delicious smelling appetizers to be delivered right when the educational portion of the evening was ending...
Between delivering appetizers and staying in the same venue, we managed to change things around and we ended up having a majority of people stick around for at least a cup of coffee and some food after.