The town's annual festival had drawn modest crowds for over a decade. ______, a single viral social media post about the festival led to…

The passage describes a straightforward sequence in time: a decade of modest attendance, followed by a specific event that changed the pattern. Then correctly signals this simple temporal progression. Namely introduces a specific example of something just named, which doesn't fit this shift between two different time periods. In contrast could work to signal a shift, but it typically frames a direct comparison between two parallel ideas instead of a sequential turning point marked by a specific triggering event, which then captures more precisely here. Specifically would frame the second sentence as a more detailed elaboration of the first's general claim, not a change in circumstance.