Sites like Meetup.com and Eventbrite.com are designed with the key intent of helping people organize and discover offline group meetings and social gatherings, in places all around the world. Search Eventbrite.com to find an event that appeals to you, or use Meetup.com to join or create a group with people near you who share your interests.
If you think language barriers might be an issue, don’t worry! Plenty of world travelers organize English language meetups regardless of the native language of the country.
Facebook is another useful tool to aid your social efforts while traveling alone. Joining groups or events for social activities that interest you is a key way to get the scoop on what’s happening in your city.
Of course, your hostel is always a great way to meet other solo travelers, too.