Dick Costolo has stepped down as CEO of Twitter after five years on the job, relinquishing his post amid longstanding problems including sluggish growth and frequent abusive comments posted to the social network. Twitter cofounder Jack Dorsey, who is also CEO of Square, will be interim CEO when Costolo steps down on July 1.

When Twitter finds a permanent replacement, he or she will face several big challenges. Here are four of them:

1. Defining what Twitter is

Perhaps the biggest problem Twitter has is that many people who aren’t tech enthusiasts still don’t understand what it’s for or why they should use it. For every occasion Twitter is referred to as a social network, it’s also identified as a news source, a publishing system, a feed of real-time events and a micro blog. Perhaps it’s all those things, but that doesn’t help sell it to people who aren’t yet on the service. If it’s a social network, why use it when Facebook’s around? If it’s a micro blog, why not use a proper blog like Tumblr instead?

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