Share a Feed Element (in Version 39.0 and Later)

// Define the FeedItemInput object to pass to postFeedElement
ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
feedItemInput.subjectId = 'me';
ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
textSegmentInput.text = 'Look at this post I'm sharing.';
// The MessageBodyInput object holds the text in the post
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
messageBodyInput.messageSegments.add(textSegmentInput);
feedItemInput.body = messageBodyInput;

ConnectApi.FeedEntityShareCapabilityInput shareInput = new ConnectApi.FeedEntityShareCapabilityInput();
shareInput.feedEntityId = '0D5R0000000SEbc';
ConnectApi.FeedElementCapabilitiesInput feedElementCapabilitiesInput = new
ConnectApi.FeedElementCapabilitiesInput();
feedElementCapabilitiesInput.feedEntityShare = shareInput;
feedItemInput.capabilities = feedElementCapabilitiesInput;
// Post the feed item.
ConnectApi.FeedElement feedElement =
ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), feedItemInput);