Hi Patrick and welcome to the community,
1. We reviewed this api on Friday and found that the ID of the newly created SocialObject was not being returned in the response xml; this has been corrected and the fix will be available after the next release. In the meantime, you can use the GetSocialObjects api to get the n most recently created SocialObjects and interrogate that.
2. How are you getting a handle on this GUID? We currently provide two ways of fetching a User: By int and by encrypted guid (this is the User's TP_SSID cookie):
http://services.theport.com/REST/V1/Users/{Int}?devkey=
or
http://services.theport.com/REST/V1/Users/{TP_SSID}?devkey=
Can you get a handle on the TP_SSID cookie?
3. We also looked into this issue on Friday; it looks as if some of the Users in the community contain an e-mail address and some do not. The example call that you provided via e-mail returns some Users with and some without. I suspect that this is an SSO issue; I will follow up with our SSO architect to see if we can get a handle on this.