Facebook (FB -2.1%) roundup: 1) Pivotal Research thinks Q1 results indicate Facebook's FBX ad targeting platform (thus far limited to PCs) is growing slower than hoped. The firm adds some of FBX's top ad agency clients are growing slower than expected. 2) Nielsen estimates U.S. visitors to Facebook's site fell 11M Y/Y in March to 142M. Facebook estimates its North American MAUs (inc. app users) rose 12M Y/Y in Q1 to 195M, but only 2M Q/Q. 52M users were added Q/Q in other markets. 3) Facebook ads can now be sent based on a user's app installs and in-app purchases. 4) Instagram has added a photo-tagging feature; AllThingsD thinks brands could make use of it. Shares seem to be selling off today in sympathy with LinkedIn.