Do the followers who signed up while Kravitz was at PhoneDog have a choice? Or, if they're unhappy with the switch, do they have to un-follow PhoneDog and re-follow Kravitz? If they do, can PhoneDog keep their contact information to send marketing material later, or should opting out of PhoneDog's Twitter feed (this one, anyway) also count as a do-not-contact notice?
Conclusion: Big mess
There's not any clean way to make the division any more than there is a way for a divorcing couple to divide up the friends between them. Putting all the football-watching friends in one pile and football haters in another will split a lot of couples such that they can no longer socialize as a couple with either of the divorcees.
That's not fair to them, even assuming they accept being assigned one spouse or the other as their officially sanctioned friend and agree to remain on Team Spouse 1 or Team Spouse 2 and obey all the policies, expectations and bylines applied thereto in perpetuity. (Few adults would go along with that without complaint, especially if the entities reassigning the loyalty of other people are faceless tweeters rather than flesh-and-blood friends whose problems deserve a little indulgence and understanding.
Even the before-and-after question is more complicated than it looks.
Companies that expect to grow by marketing through social networks often advertise for social-media producers, coordinators or content producers with a proven ability to gather an audience through effective promotion and content. What they want is people who are already tweeting and Facebook-updating and Slashdotting and Farking and writing regularly for any other community-contribution content-distribution network.
That means they come with baggage – followers that are considered a major advantage that helps social mediators get hired. When they leave, is it fair to have all the assets they brought with them to the company have to stay behind? That eliminates one of the primary reasons the company hired that employee in the first place, making it even harder for that employee to find another gig in a job market that's going on the third year of painful constriction.
That's hardly fair to the employee, or to the audience, which would probably not stay with whatever replacement the employer chooses to take over the tweeting, probably without knowing the audience, or even the topic, well.
My inclination is to give PhoneDog a full list of the followers who were active the day Kravitz left, let Kravitz keep the account and all the followers – to avoid pissing off everyone who thinks well of either the company or the tweeter. Kravitz, and any other tweeters, bloggers or other social-media personalities, should have to give up any pseudonym or tag line or column title created for the previous employer, but face no restriction on working under their own names.