Whether you bought or used other available options to get your 500 Twitter followers, tweeting often is the only way that will ensure the numbers keep rising with minimum unfollowing. Most tweeps ignore this though it seems like common sense on this social media platform. If you have a huge following, people will come to your timeline to at least assess the content you are posting to ascertain why you command such a huge following. If the last tweets were posted many months ago, no one would want to follow such a dormant account. Those already following you will unfollow after discovering the same. To attract and retain your existing followers, consider tweeting all the time. Caution should be taken with this step too as filling the timeline with unrelated content in a great undoing on your part. With followers across various time zones, you can opt to tweet at any time even if it is midnight at your local time. The essence is keeping your account alive and full of engagement. With all these considerations, you will attract over 500 twitter followers within a short period.