It was sort of inevitable given Facebook’s monster growth over the last few years, but April 2008 was the milestone: Facebook officially caught up to MySpace in terms of unique monthly worldwide visitors, according to data released by Comscore and shown above. Both services are attracting around 115 million people to their respective sites each month.

Most of Facebook’s user growth, however, has been in international markets – MySpace is still dominates Facebook in the U.S. market, with 72 million monthly uniques. Facebook has 36 million monthly uniques, up from 23 million a year ago.

Facebook added 75 million monthly uniques over the last twelve month, but just 13 million of those visitors are located in the U.S. MySpace added 5 million U.S. uniques during that period – at this rate it will take 4+ years for Facebook to catch up to MySpace in the U.S. market.

There’s a real question about how valuable all these international users are from an advertising standpoint. We’ll be publishing our thoughts on that next week.