This Sunday I sat on the steps of the church and observed the activity of the piazza at various points of the day. I sat on the steps at noon, and watched as tours circled around the fountains. I returned in the late afternoon, and saw Italian youths/preteens meet up with their friends. I ended the night people-watching in the piazza around 11 PM, and noticed that there were hardly any people out compared to how many there were in the morning.
During the daytime, there were consistent trends of how people of different age groups use the space. Older people stick to the periphery, unless they are part of tours around the city and will spread out around the piazza. Younger people tend to have a presence around the entire piazza, with groups congregating on the steps or seated outside cafes. These general traffic patterns hold true for their respective age groups, with preteens usually passing through, and older groups of people lingering for more time in the piazza.
Since I sat out on a Sunday, Corso Vannucci was more crowded than it usually is, seeing as the national museums are free on the first Sunday of the month. More people were out during the daytime when it was sunnier, and they usually stayed on Corso Vannucci in front of the shops and restaurants there. At night, there were fewer people out, probably because it is the chilliest part of the day.