1) Every social service publishes atom feeds. Feeds contain entries and/or other Feeds. Feeds within feeds is the tree. Every feed and entry in a feed is addressable by its unique URI.

2) Every service supports user-created search feeds. In effect, this is a “track” feed with sql-like where predicates. Users can create and delete as desired.

3) Feed Entries must contain at least the following fields (schema):
EntryURI (unique entry URI)
ReplyURI (null or the entry being replied to)
To (users or user-feeds (i.e. groups) to send msg to)
From (user URI who sent this entry)
UTCDate (msg date in UTC)
Comment (msg string)
Attachments (null, Entry URIs)

4) Every service must support at least one User Attachment feed to store attachments (i.e. pictures, docs, etc). Service specified storage limits. Attachments pushed from the client are stored as separate Entries with a URI as normal. The Attachments field then contains the blob data as base64 wrapped in an XML infoset. This could be an opt-in feature of the service if they don’t support user space.

5) EntryURI must be unique. The EntryURI serves as both the path to the entry as well as the key across the Net.

6) Services must support RESTfull DB queries (i.e. Astoria like).

What this provides:
1) Simple and common. All entries from all services are static and addressable.
2) REST and ATOM are the lowest common denominator. No other standards need to be made. All platforms supported by nature of atom and REST. No proprietary APIs required.
3) Minimum set of fields (above) are requirement. Order not important and any derived schemas supported. Clients just ignore unknown fields.
4) Client frameworks can aggregate and relate feed collections anyway they want.
5) Attachments are simple links to attachment entries stored at the service. By convention, the client software pushes attachments to the user’s Attachment Feed that is addressable by others. Alternatively, if the user manually supplies attachment URL(s), they are passed in place and no attachments are stored at service.