Jive's social business platform has been designed from scratch as an enterprise social network.
Jive has separate clients for iPhones and iPads, but both are highly rated (~4.5 out of 5). The Android version ranks 3.5 out of 5.
Microsoft's document-centric SharePoint is a major player in enterprise collaboration, but isn't very social or mobile.
Though well-reviewed options like Infragistic's SharePlus exist on both iOS and Android.
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