Many geeks remember the days of AOL and AIM when all the good usernames were taken so everyone had to get creative with their screen names. Long strings of numbers and underscores were the norm.
A different standard has emerged on Twitter. Many successful users use their first and last names concatenated into one long string. Unfortunately, some people find their first and last name taken (especially people with common names), and resort to a name with underscores and numbers.
So, while my Twitter handle is @danzarrella, the next Dan Zarrella to join Twitter might pick @dan_zarrella.
This is a bad idea, particularly if you're trying to build an account with lots of followers.
Using data from Twitter Grader (a database with close to 2 million Twitter users), I took a look at the relationship between the presence of underscores and numbers in usernames and average follower numbers.
The results are pretty much what you'd expect, if only surprising in how clear-cut they are.