Formatting dates on X axis in ggplot2

To show months as Jan 2017 Feb 2017 etc:

scale_x_date(date_breaks = "1 month", date_labels =  "%b %Y") 

Angle the dates if they take up too much space:

theme(axis.text.x=element_text(angle=60, hjust=1))

Can you use date as a factor?

Yes, but you probably shouldn't.

...or should you use as.Date on a date column?

Yes.

Which leads us to this:

library(scales)
df$Month <- as.Date(df$Month)
ggplot(df, aes(x = Month, y = AvgVisits)) + 
  geom_bar(stat = "identity") +
  theme_bw() +
  labs(x = "Month", y = "Average Visits per User") +
  scale_x_date(labels = date_format("%m-%Y"))

in which I've added stat = "identity" to your geom_bar call.

In addition, the message about the binwidth wasn't an error. An error will actually say "Error" in it, and similarly a warning will always say "Warning" in it. Otherwise it's just a message.