Among some Twitter uses there is a push to increase numbers, mostly coming from people with a message to push or a service to promote.
Regular users quickly learn to spot the spammers and promoters and either ignore them or report them to @spam, who monitor any suspicious or unacceptable activity. The dictum that a user should follow, out of some kind of duty or politeness, every person who follows them, is impractical in reality - the number of posts received would be enormous and any hope of maximizing the real potential of Twitter as a communication and information tool would be wasted.
At the time of writing, Britney Spears is the celebrity tweeter with the highest patronage, with 1,091,719 people following her every word. But up there with her are NBC compere Jimmy Fallon (778,842), US President Barack Obama (749,682), singer John Mayer (724,101), actress Demi Moore (679,282) famous person Paris Hilton (655,755) and cyclist Lance Armstrong (646,925). (9)
In all cases the celebrity's followee lists are very much shorter, suggesting that little interaction occurs other than with a close circle of friends. Tweets received would be very much a flow of fan mail, impossible to interact with.