Did you forget to log out of Facebook at your friend's house, did you accidentally click on something you shouldn't have, or has your account been hacked?
I can't answer these questions without actually seeing what's happening on your profile, but I can offer some advice. Here are four things you should try in order to clean up your Facebook account.
Reclaim your account
If you can no longer login to Facebook, you'll want to follow these instructions from the Facebook Help Center:
My account is hacked. If your account is sending out spam (ex: advertisements or suspicious links) or was taken over by someone else, secure it here.
After you've navigated to facebook.com/hacked, click the blue "Continue" button and follow the instructions.
Change your Facebook password
It's possible your Facebook woes are coming from the result of a phishing scam. Someone may have created a fake website that looks like Facebook or another online service you visit and tricked you into logging in. Their goal was to steal your password and other account credentials, and they may have succeeded.
In this case, you should change your password on Facebook. If you don't know how to do so, you can refer to the following guide: How to change your Facebook password.
If changing your password fixes your Facebook problems, you should change your password for all your other services too, especially if you use the same password for them as you previously used on Facebook. If this doesn't fix the problem, try the next step.
Remove unwanted Facebook apps
It's possible your Facebook woes are coming from a rogue app that you accidentally installed or were tricked into installing. Every Facebook app has certain permissions to your account. Some of these permissions you can modify, while others you cannot.
Your best bet is to remove all the Facebook apps you find suspicious. If you don't know how to do so, you can refer to the following guide: How to clean up your Facebook apps.
If cleaning out your apps fixes your Facebook problems, tell your friends they should do the same (chances are the app asked your friends to install it as well). If this doesn't fix the problem, try the next step.
Get some security software and run a virus scan
It's possible your Facebook woes are coming from some sort of malware, be it a keylogger, a trojan, or some other type of virus. Even if you think your computer is clean, it can't hurt to check.
I recommend Microsoft Security Essentials –it's free and gets the job done very well. Another good one is Malwarebytes. Other free alternatives include Avira and Avast.
The aforementioned security programs are for Windows. If you have a Mac, try using the antivirus from Sophos.
After running the virus scan, clean out whatever the program detects. If you're not sure about what it found, ask a friend who might.