Oblonsky is presented as a person who is everybody's friend. He's not an especially good person, though. He cheats on his wife. He makes a poor deal selling his land. He's not particularly good at his job. The reason he survives is because everyone likes him, but he's not sharp enough (or ruthless enough) to use this likability to his advantage. You probably know someone like him.
Levin, on the other hand, doesn't have many friends. He prefers his farm to the city. He tends to focus on his own affairs, meaning he takes little interest in the affairs of others. He doesn't like small talk. Oblonsky is happy to talk to him, where few others will tolerate him.
In other words, Levin has few friends, while Oblonsky has many. The relationship is asymmetric, but it works because Levin has no one else to turn to. Oblonsky could be smarter, more sober, or better at business, but as long as he's friendly, Levin will be his friend.