following up -
if your users can't remember that they've signed up previously, well, best of luck to them in general ;)
much as you described, i'm planning on giving users the option to link additional accounts once they have signed in by one means or another.
but as far as cross-checking, there's only so much you can do. many social network APIs do indeed provide email addresses (once you've busted in through OAuth) but these may be accessible only if a user has elected to make his/her address public, which is not guaranteed.
also not guaranteed is that the user used the SAME email address for each social network account, so even if you manage to retrieve an address it may or not be of any use to you.
finally, if you find matching email addresses via such means, it might be advisable to prompt the user to link accounts rather than assume he/she wants this done automatically. some people like to maintain multiple personalities. i.e. "it looks like you are also signed up with twitter - do you want to link your accounts? it will make your life seem worth living."
you might consider offering incentives to link user accounts or to provide an email address (up to you of course to figure out what these might be, based on the functionality of your website).
solution i am working on, database-side, is to maintain multiple accounts and then if link information is discovered by various means, said link is indicated in a lookup table.
an alternative is once you find a link, attempt to combine all relevant entries for the multiple accounts into one account entity - all i can say about this latter approach is that i would do so with caution as there could be a formidable level of complexity depending on the user's activity level and the complexity of your database schema.
in my (mental/actual) namespace a user who registers the old-fashioned way has a 'standard' account and one who uses a social network has an 'alias' account. then the goal becomes to define where the alias is supposed to point, i.e. create the lookup such that a subsequent login via either means retrieves the relevant information for both accounts (with a preference for displaying personal data for the 'standard' account).
btw i figured out how to make twitter OAuth behave since my last post - you can look at my other answers for details if you're interested.
JB
hi matt,
i'm working on the same problem right
now.
assuming the user starts with regular
site account (which is not
necessarily safe to assume if he sees
all the pretty "connect with XXX
network" buttons!!!), you can use
either OAuth or the javascript APIs
(facebookConnect or @anywhere -
haven't fully figured out the latter
yet and i'm not sure I recommend it as
I don't think it provides as rich an
API as do the backend libraries) to
login to the other sites.
the APIs should return certain
information after a successful
login/redirect from the social network
- such as the user ID and an ACCESS TOKEN which you can then store in your
database in some capacity associating
your 'actual' application user with
the ID of the social network.
when the user returns to the site, you
can then
1 verify cookies set by the social
network services (various schemes
typically verifying a signature, based
on sha1 or md5 hash of your
application data - by which i mean the
data you get when you register your
app with twitter/facebook, typically a
consumer key, application ID, etc. -
with the received cookies) so you know
the user has logged in with the social
network
2 find your database entry association
as described above
3 login your user manually based on
the assumption that facebook/twitter
connection is secure.
caveat: this is only as secure as your
implementation (or as secure as
facebook/twitter's implementations, if
you prefer...)
although twitter's OAuth does not
currently seem to work quite right,
their general description of the
process is pretty informative:
http://dev.twitter.com/pages/auth
good luck.
J