Meetup.com is a great website that offers common interest group activities. You can join meetup groups based on hobbies, activities, location and more. It was helpful for me when I first moved to Boston. There is also an organization called Boston Young Professionals, which hosts several social activities in the Boston area for young professionals; their website is: http://www.bostonypa.com/.
There are tons of bars in The Harvard Sq area, as well as in Porter, and Davis.
The Back Bay is another option too.
The Back Bay tends to attract more professionals; thus making it a better choice for what you are looking for.
There a few sites that offer fun things to do for single professionals. Also you should check out a few local bars, restaurants, clubs in Boston. Friday and Saturday nights are usually best nights to go out and meet people.