If you use Hootsuite, then you may have noticed the “Bulk Message Uploader” link in the scheduling pop-up. I never looked into how it works. However, Lightspan Digital posted a great step-by-step instruction on how to use it. It even includes a list of errors to avoid.
A great post to check out if you have a Hootsuite account.
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Source: Scheduling Tweets: How to Use Hootsuite’s Bulk Uploader