While monthly visitor numbers don’t tell the whole story about the health of a social network, in this case it was more of the same for top social networks Facebook and MySpace, and microblogging service Twitter in the US, according to Compete.

Facebook grew relatively slowly in September according to Compete, by 1.93% to 125 million. MySpace, however, fell 9.66 percent to 50.1 million. Twitter, stayed at around 23.5%. None of the sites have seen any serious growth for the last few months according to these numbers, although Facebook and Twitter both grew quickly earlier this year. MySpace had actually been growing through June, when it reached 61.0 million.

Compete’s numbers tend to run on the high side compared to other analytics companies, although similar directional trends are usually visible across multiple results. You can see the same trend for September in Hitwise’s market share results from yesterday. The internal data that Facebook makes publicly available showed an 8.6% increase in September to 88.3 million monthly actives. Neither comScore nor Quantcast have released their September data yet.