Facebook and Twitter have announced changes to their services, making each social network look a little more like the other. 

Twitter announced changes to its terms of service Thursday, one of which could lead to advertising on the microblogging service.

"In the terms, we leave the door open for advertising," Biz Stone wrote on the official Twitter blog.

Twitter, a free microblogging and social networking service, has been criticized for lacking a business model. Facebook is also free, but gets revenue from advertising.

Also on Thursday, Facebook announced a new feature that would let users tag their friends in status updates, similar to the way friends can be tagged in photos, videos and notes.

To use the feature, which Facebook said would roll out over the next few weeks, users will have to type the "@" symbol in their posts. A drop-down menu will appear listing the user's friends. The "@" symbol won't appear in the post itself; only the person's name and a link to his or her profile will show up.

Twitter users have long used the "@" symbol to reply to other users or mention them in their tweets.

Facebook also publicly launched a "lite" version of its site Friday, a stripped-down service that excludes games and other apps. Facebook Lite concentrates on status updates and shared links, similar to the bare-bones approach of Twitter.

Facebook Lite had previously been available only to beta testers.

In a related development Thursday, Facebook also opened the source code of the software framework that powers FriendFeed, an online, real-time aggregator of Twitter feeds and other social media services, social-networking websites and blogs.

Facebook acquired FriendFeed and its 12 employees in August.