If you have already linked your accounts, you'll have the option to share to Facebook/Twitter on each check-in. Tap the Facebook or Twitter icons until they are highlighted to share.
Please note: settings are sticky, which means that if you tap to share on one check-in, we'll remember that decision for your next check-in. If you decide not to share to one or both networks, simple tap the icons again until they are no longer blue. The next time you check in, they will remain off. And so on.
Linking your accounts
To link your Foursquare account to Twitter or Facebook, or to adjust your sharing settings with those networks, tap the 'me' (face icon) in the upper right corner of the screen. Tap 'My Settings,' then 'Account Settings,' and finally 'Facebook & Twitter.'
From here, you'll have the option to connect your accounts, and to choose what types of content you do and do not share.
To disconnect your accounts, tap 'disconnect' from the same screen as above.
Why isn't my check-in, tip, photo, etc. posting to Twitter/Facebook?
Your accounts need to be connected at foursquare.com/settings and you might want to check your Facebook settings to make sure you're allowing Foursquare to post. If you *already* linked your accounts, you should disconnect the account at foursquare.com/settings, sign out of Facebook (and sign out of Foursquare and Facebook on your device), then reconnect the accounts and choose the settings you want: send badges, mayorships, and check-ins to Facebook or Twitter. Facebook has a rate limit of 10 posts in 48 hours from apps like Foursquare.
For Twitter, follow the instructions above to disconnect and reconnect the accounts. The same settings apply as above (tweet mayorship, badge, check-in)
If you're STILL having trouble, please see our thorough troubleshooting guide here.