I've been developing web and iPhone apps for a while and I usually recommend to first use the provided sharing mechanisms. They provide great flexibility and as well don't require your app to request permissions on external networks. Using the iOS SDK sharing components will also save you precious development time.
The most important part is the content that will be shared. On that side, make sure all the content has a unique URL, and can be properly consumed on all plateforms, desktop and mobile.
Pay a particular attention to the head meta tags of those pages. Open graph and Twitter tags are not optional anymore and help driver a better visibility on the networks your content is being shared onto.
Don't forget to track with an appropriate analytics tool how your users use the sharing module. Then you can evaluate the pertinence of building something custom.