A brand new publish service is available from LRInstagram.com that will allow you to export from Lightroom directly to Instagram complete with captions, hash-tags, and proper cropping. The plugin is compatible with Adobe Lightroom CC, or LR 3.0 and later
Set up is pretty simple. First download the plugin and install using the Lightroom Plug-In manager. Then set-up the publish service by authenticating with your Instagram account and configuring options for captioning, hash-tagging, and auto-cropping if desired. You can also include output sharpening and a watermark.
Captions and hashtags can be taken directly from a newly expanded Instagram metadata section within Lightroom's regular metadata pane.
The configurable options for captioning your post using LR metadata include: None, Title, Caption, or #Hashtag as well as combinations of "Title + #Hashtag" or "Caption + #Hashtag" making it very simple to create your post with just the right text. I use Caption + Hashtag for my posts.
Because the plug-in creates a Lightroom publish service, the connection is bi-directional and you can sync comments back into Lightroom from Instagram.
A post-processing action can also be configured as part of the publish service that will force the cropping of your picture to one of the supported Instagram aspect ratios that ranges between 4:5 and 1.91:1.00. (You may wish to manually crop if you are including a watermark as it may get trimmed in the process.)
The shareware plug-in is licensed via a $10 PayPal payment using the plug-in manager in Lightroom.